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/DocumentsPage.tsx

@@ -1,6 +1,5 @@
 import { useRef, useState, useEffect, useCallback } from "react";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
-import Nav from "../components/Nav";
 import {
   listDocuments,
   uploadDocument,
@@ -707,7 +706,6 @@ export default function DocumentsPage() {
 
   return (
     <>
-      <Nav />
       <div style={{ padding: 32, maxWidth: 960, margin: "0 auto" }}>
         <h1>Documents</h1>