feat(frontend): add shared client libraries and UI primitives

Introduce API helpers, types, hooks, design-system components, and the authenticated app shell.

frontend/src/components/ui/textarea.tsx

@@ -0,0 +1,19 @@
+import * as React from "react";
+import { cn } from "@/lib/utils";
+
+const Textarea = React.forwardRef<
+  HTMLTextAreaElement,
+  React.TextareaHTMLAttributes<HTMLTextAreaElement>
+>(({ className, ...props }, ref) => (
+  <textarea
+    className={cn(
+      "flex min-h-[120px] w-full rounded-lg border border-input bg-card px-3.5 py-2.5 font-sans text-sm font-medium leading-normal outline-none transition-colors placeholder:font-sans placeholder:font-medium placeholder:text-muted-foreground focus-visible:ring-2 focus-visible:ring-ring disabled:cursor-not-allowed disabled:opacity-50",
+      className,
+    )}
+    ref={ref}
+    {...props}
+  />
+));
+Textarea.displayName = "Textarea";
+
+export { Textarea };