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/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();