feat: Add skeleton loaders for improved loading states in studies and participants pages

- Implemented skeleton components for studies list, study details, and participants table
- Enhanced loading experience by replacing simple "Loading..." placeholders with detailed skeleton loaders
- Created reusable skeleton components in `src/components/ui/skeleton.tsx`
- Updated studies and participants pages to use new skeleton loaders

src/app/dashboard/studies/[id]/edit/page.tsx

@@ -7,6 +7,7 @@ import { PageHeader } from "~/components/layout/page-header";
 import { PageContent } from "~/components/layout/page-content";
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from "~/components/ui/card";
 import { use } from "react";
+import { CardSkeleton } from "~/components/ui/skeleton";
 
 export default function EditStudyPage({ params }: { params: Promise<{ id: string }> }) {
   const router = useRouter();
@@ -29,7 +30,27 @@ export default function EditStudyPage({ params }: { params: Promise<{ id: string
   }
 
   if (isLoadingStudy) {
-    return <div>Loading...</div>;
+    return (
+      <>
+        <PageHeader
+          title="Edit Study"
+          description="Loading study details..."
+        />
+        <PageContent className="max-w-2xl">
+          <Card>
+            <CardHeader>
+              <CardTitle>Study Details</CardTitle>
+              <CardDescription>
+                Please wait while we load the study information.
+              </CardDescription>
+            </CardHeader>
+            <CardContent>
+              <CardSkeleton />
+            </CardContent>
+          </Card>
+        </PageContent>
+      </>
+    );
   }
 
   if (!study) {

src/app/dashboard/studies/[id]/page.tsx

@@ -13,6 +13,7 @@ import { StudyParticipants } from "~/components/studies/study-participants";
 import { StudyMembers } from "~/components/studies/study-members";
 import { StudyMetadata } from "~/components/studies/study-metadata";
 import { StudyActivity } from "~/components/studies/study-activity";
+import { StudyDetailsSkeleton } from "~/components/ui/skeleton";
 
 export default function StudyPage({ params }: { params: Promise<{ id: string }> }) {
   const router = useRouter();
@@ -24,7 +25,28 @@ export default function StudyPage({ params }: { params: Promise<{ id: string }>
   const { data: study, isLoading: isLoadingStudy } = api.study.getById.useQuery({ id });
 
   if (isLoadingStudy) {
-    return <div>Loading...</div>;
+    return (
+      <>
+        <PageHeader
+          title="Loading..."
+          description="Please wait while we load the study details"
+        />
+        <PageContent>
+          <Tabs defaultValue="overview" className="space-y-4">
+            <TabsList>
+              <TabsTrigger value="overview">Overview</TabsTrigger>
+              <TabsTrigger value="participants">Participants</TabsTrigger>
+              <TabsTrigger value="members">Members</TabsTrigger>
+              <TabsTrigger value="metadata">Metadata</TabsTrigger>
+              <TabsTrigger value="activity">Activity</TabsTrigger>
+            </TabsList>
+            <TabsContent value="overview">
+              <StudyDetailsSkeleton />
+            </TabsContent>
+          </Tabs>
+        </PageContent>
+      </>
+    );
   }
 
   if (!study) {

src/app/dashboard/studies/[id]/participants/[participantId]/edit/page.tsx

@@ -9,6 +9,7 @@ import { ParticipantForm, type ParticipantFormValues } from "~/components/partic
 import { use } from "react";
 import { useToast } from "~/hooks/use-toast";
 import { ROLES } from "~/lib/permissions/constants";
+import { CardSkeleton } from "~/components/ui/skeleton";
 
 export default function EditParticipantPage({
   params,
@@ -50,7 +51,27 @@ export default function EditParticipantPage({
   }
 
   if (isLoading) {
-    return <div>Loading...</div>;
+    return (
+      <>
+        <PageHeader
+          title="Edit Participant"
+          description="Loading participant information..."
+        />
+        <PageContent>
+          <Card>
+            <CardHeader>
+              <CardTitle>Participant Details</CardTitle>
+              <CardDescription>
+                Please wait while we load the participant information.
+              </CardDescription>
+            </CardHeader>
+            <CardContent>
+              <CardSkeleton />
+            </CardContent>
+          </Card>
+        </PageContent>
+      </>
+    );
   }
 
   if (!study || !participant) {

src/app/dashboard/studies/page.tsx

@@ -7,15 +7,12 @@ import { PageContent } from "~/components/layout/page-content";
 import { Button } from "~/components/ui/button";
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from "~/components/ui/card";
 import { Plus as PlusIcon } from "lucide-react";
+import { StudyListSkeleton } from "~/components/ui/skeleton";
 
 export default function StudiesPage() {
   const router = useRouter();
   const { data: studies, isLoading } = api.study.getMyStudies.useQuery();
 
-  if (isLoading) {
-    return <div>Loading...</div>;
-  }
-
   return (
     <>
       <PageHeader
@@ -31,8 +28,9 @@ export default function StudiesPage() {
         </Button>
       </PageHeader>
       <PageContent>
-        <div className="grid gap-6">
-          {!studies || studies.length === 0 ? (
+        {isLoading ? (
+          <StudyListSkeleton />
+        ) : !studies || studies.length === 0 ? (
           <Card>
             <CardHeader>
               <CardTitle>No Studies</CardTitle>
@@ -42,7 +40,8 @@ export default function StudiesPage() {
             </CardHeader>
           </Card>
         ) : (
-            studies.map((study) => (
+          <div className="grid gap-6">
+            {studies.map((study) => (
               <Card
                 key={study.id}
                 className="hover:bg-muted/50 cursor-pointer transition-colors"
@@ -60,9 +59,9 @@ export default function StudiesPage() {
                   </div>
                 </CardContent>
               </Card>
-            ))
-          )}
+            ))}
           </div>
+        )}
       </PageContent>
     </>
   );

src/components/studies/study-participants.tsx

@@ -18,6 +18,7 @@ import { ROLES } from "~/lib/permissions/constants";
 import { Switch } from "~/components/ui/switch";
 import { Label } from "~/components/ui/label";
 import { useState } from "react";
+import { TableSkeleton } from "~/components/ui/skeleton";
 
 interface StudyParticipantsProps {
   studyId: number;
@@ -38,7 +39,21 @@ export function StudyParticipants({ studyId, role }: StudyParticipantsProps) {
     .includes(role.toLowerCase());
 
   if (isLoading) {
-    return <div>Loading...</div>;
+    return (
+      <Card>
+        <CardHeader>
+          <div className="flex items-center justify-between">
+            <div>
+              <CardTitle>Study Participants</CardTitle>
+              <CardDescription>Loading participants...</CardDescription>
+            </div>
+          </div>
+        </CardHeader>
+        <CardContent>
+          <TableSkeleton />
+        </CardContent>
+      </Card>
+    );
   }
 
   return (

src/components/ui/skeleton.tsx

@@ -1,4 +1,5 @@
 import { cn } from "~/lib/utils"
+import { Card, CardContent, CardHeader } from "~/components/ui/card"
 
 function Skeleton({
   className,
@@ -6,10 +7,132 @@ function Skeleton({
 }: React.HTMLAttributes<HTMLDivElement>) {
   return (
     <div
-      className={cn("animate-pulse rounded-md bg-primary/10", className)}
+      className={cn("animate-pulse rounded-md bg-muted", className)}
       {...props}
     />
   )
 }
 
-export { Skeleton }
+function TableRowSkeleton() {
+  return (
+    <div className="flex space-x-4 p-4">
+      <Skeleton className="h-5 w-[20%]" />
+      <Skeleton className="h-5 w-[15%]" />
+      <Skeleton className="h-5 w-[30%]" />
+      <Skeleton className="h-5 w-[35%]" />
+    </div>
+  )
+}
+
+function TableSkeleton() {
+  return (
+    <div className="space-y-3">
+      <div className="flex space-x-4 p-4 border-b">
+        <Skeleton className="h-4 w-[20%]" />
+        <Skeleton className="h-4 w-[15%]" />
+        <Skeleton className="h-4 w-[30%]" />
+        <Skeleton className="h-4 w-[35%]" />
+      </div>
+      {[...Array(5)].map((_, i) => (
+        <TableRowSkeleton key={i} />
+      ))}
+    </div>
+  )
+}
+
+function CardSkeleton() {
+  return (
+    <div className="p-6 space-y-4">
+      <Skeleton className="h-7 w-[40%]" />
+      <Skeleton className="h-4 w-[60%]" />
+      <div className="pt-4">
+        <Skeleton className="h-4 w-[25%]" />
+      </div>
+    </div>
+  )
+}
+
+function StudyListSkeleton() {
+  return (
+    <div className="space-y-6">
+      {[...Array(3)].map((_, i) => (
+        <div key={i} className="rounded-lg border bg-card">
+          <CardSkeleton />
+        </div>
+      ))}
+    </div>
+  )
+}
+
+function StudyDetailsSkeleton() {
+  return (
+    <div className="space-y-6">
+      {/* Overview Card */}
+      <Card>
+        <CardHeader>
+          <Skeleton className="h-7 w-[150px] mb-2" />
+          <Skeleton className="h-4 w-[250px]" />
+        </CardHeader>
+        <CardContent>
+          <div className="grid gap-4 sm:grid-cols-2">
+            <div>
+              <Skeleton className="h-4 w-[100px] mb-1" />
+              <Skeleton className="h-5 w-[150px]" />
+            </div>
+            <div>
+              <Skeleton className="h-4 w-[100px] mb-1" />
+              <Skeleton className="h-5 w-[200px]" />
+            </div>
+          </div>
+        </CardContent>
+      </Card>
+
+      {/* Stats Cards */}
+      <div className="grid gap-4 md:grid-cols-3">
+        {[...Array(3)].map((_, i) => (
+          <Card key={i}>
+            <CardHeader className="flex flex-row items-center justify-between space-y-0 pb-2">
+              <Skeleton className="h-4 w-[120px]" />
+              <Skeleton className="h-4 w-4" />
+            </CardHeader>
+            <CardContent>
+              <Skeleton className="h-7 w-[60px] mb-1" />
+              <Skeleton className="h-4 w-[100px]" />
+            </CardContent>
+          </Card>
+        ))}
+      </div>
+
+      {/* Activity Card */}
+      <Card>
+        <CardHeader>
+          <Skeleton className="h-7 w-[150px] mb-2" />
+          <Skeleton className="h-4 w-[200px]" />
+        </CardHeader>
+        <CardContent>
+          <div className="space-y-4">
+            {[...Array(3)].map((_, i) => (
+              <div key={i} className="flex gap-4">
+                <Skeleton className="h-8 w-8 rounded-full" />
+                <div className="flex-1 space-y-2">
+                  <Skeleton className="h-4 w-[150px]" />
+                  <Skeleton className="h-4 w-[250px]" />
+                  <Skeleton className="h-3 w-[100px]" />
+                </div>
+              </div>
+            ))}
+          </div>
+        </CardContent>
+      </Card>
+    </div>
+  )
+}
+
+export { 
+  Skeleton,
+  TableRowSkeleton,
+  TableSkeleton,
+  CardSkeleton,
+  StudyListSkeleton,
+  StudyDetailsSkeleton,
+}