feat: add UI components including Skeleton, Slider, Toaster, Switch, Table, Tabs, Textarea, Toggle Group, Toggle, Tooltip, and User Avatar

- Implemented Skeleton component for loading states.
- Added Slider component using Radix UI for customizable sliders.
- Created Toaster component for notifications with theme support.
- Developed Switch component for toggle functionality.
- Introduced Table component with subcomponents for structured data display.
- Built Tabs component for tabbed navigation.
- Added Textarea component for multi-line text input.
- Implemented Toggle Group and Toggle components for grouped toggle buttons.
- Created Tooltip component for displaying additional information on hover.
- Added User Avatar component for displaying user images with fallback.
- Implemented useIsMobile hook for responsive design.
- Created API utility functions for user and thought data fetching.
- Added utility function for class name merging.
- Updated package.json with new dependencies for UI components and utilities.
- Added TypeScript configuration for path aliasing.

thoughts-frontend/components/user-avatar.tsx

@@ -0,0 +1,18 @@
+import { Avatar, AvatarFallback, AvatarImage } from "@/components/ui/avatar";
+import { User } from "lucide-react";
+
+interface UserAvatarProps {
+  src?: string | null;
+  alt?: string | null;
+}
+
+export function UserAvatar({ src, alt }: UserAvatarProps) {
+  return (
+    <Avatar>
+      {src && <AvatarImage src={src} alt={alt ?? "User avatar"} />}
+      <AvatarFallback>
+        <User className="h-5 w-5" />
+      </AvatarFallback>
+    </Avatar>
+  );
+}