Clean up unused imports and refactor type definitions

- Remove unused `cn` import from theme-selector - Remove unused `Slot`
import from badge - Remove unused `X` icon import from switch - Replace
empty interface extends with type alias in input - Replace empty
interface extends with type alias in textarea - Add "secondary" variant
to button type props - Replace "brand" variant with "default" in
client-list and invoice cards

src/components/data/client-list.tsx

@@ -102,7 +102,7 @@ export function ClientList() {
         </CardHeader>
         <CardContent className="text-center">
           <Link href="/dashboard/clients/new">
-            <Button variant="brand" className="h-12 w-full">
+            <Button variant="default" className="h-12 w-full">
               <Plus className="mr-2 h-4 w-4" />
               Add Your First Client
             </Button>
@@ -131,7 +131,7 @@ export function ClientList() {
           </div>
         </div>
         <Link href="/dashboard/clients/new">
-          <Button variant="brand" className="h-12 w-full sm:w-auto">
+          <Button variant="default" className="h-12 w-full sm:w-auto">
             <Plus className="mr-2 h-4 w-4" />
             Add Client
           </Button>

src/components/data/current-open-invoice-card.tsx

@@ -63,7 +63,7 @@ export function CurrentOpenInvoiceCard() {
               No open invoice found. Create a new invoice to start tracking your
               time.
             </p>
-            <Button asChild variant="brand">
+            <Button asChild variant="default">
               <Link href="/dashboard/invoices/new">
                 <Plus className="mr-2 h-4 w-4" />
                 Create New Invoice
@@ -133,7 +133,7 @@ export function CurrentOpenInvoiceCard() {
               View
             </Link>
           </Button>
-          <Button asChild variant="brand" size="sm" className="flex-1">
+          <Button asChild variant="default" size="sm" className="flex-1">
             <Link href={`/dashboard/invoices/${currentInvoice.id}`}>
               <Edit className="mr-2 h-3 w-3" />
               Continue