Implement shadcn/ui + Tailwind CSS UI layer

- Design token system via CSS custom properties (light/dark mode)
- Theme context hook + ThemeToggle component
- AppShell + collapsible Sidebar replace inline Nav
- LoginPage redesigned: two-column grid with hero panel
- shadcn/ui Button and Input components
- Tailwind config wired to CSS variable tokens
- All pages de-Nav'd; PrivateRoute/AdminRoute wrap with AppShell
- TypeScript passes clean (npm run typecheck)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/ProfilePage.tsx

@@ -1,7 +1,6 @@
 import { useState } from "react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { getMe, getProfile, updateProfile, type ProfileUpdate } from "../api/client";
-import Nav from "../components/Nav";
 
 export default function ProfilePage() {
   const queryClient = useQueryClient();
@@ -54,11 +53,10 @@ export default function ProfilePage() {
     mutation.mutate(payload);
   };
 
-  if (isLoading) return <><Nav /><div style={{ padding: 32 }}>Loading…</div></>;
+  if (isLoading) return <div style={{ padding: 32 }}>Loading…</div>;
 
   return (
     <>
-      <Nav />
       <div style={{ padding: 32, maxWidth: 480 }}>
         <h1>Profile</h1>