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/app/dashboard/invoices/[id]/_components/pdf-download-button.tsx

@@ -9,7 +9,7 @@ import { Download, Loader2 } from "lucide-react";
 
 interface PDFDownloadButtonProps {
   invoiceId: string;
-  variant?: "default" | "outline" | "ghost" | "icon";
+  variant?: "default" | "outline" | "ghost" | "icon" | "secondary";
   className?: string;
 }
 

src/app/dashboard/settings/_components/theme-selector.tsx

@@ -9,7 +9,6 @@ import {
   DropdownMenuTrigger,
 } from "~/components/ui/dropdown-menu";
 import { Button } from "~/components/ui/button";
-import { cn } from "~/lib/utils";
 
 export function ThemeSelector() {
   const { theme, setTheme } = useTheme();