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/forms/enhanced-send-invoice-button.tsx

@@ -7,7 +7,7 @@ import { Send, Loader2, Mail, MailCheck } from "lucide-react";
 
 interface EnhancedSendInvoiceButtonProps {
   invoiceId: string;
-  variant?: "default" | "outline" | "ghost" | "icon";
+  variant?: "default" | "outline" | "ghost" | "icon" | "secondary";
   className?: string;
   showResend?: boolean;
   size?: "default" | "sm" | "lg" | "icon";