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/AIAdminSettingsPage.tsx

@@ -1,6 +1,5 @@
 import { useEffect, useState } from "react";
 import { useQuery, useMutation } from "@tanstack/react-query";
-import Nav from "../components/Nav";
 import { getAISettings, updateAISettings, testAIConnection } from "../api/client";
 
 type Provider = "anthropic" | "ollama" | "lmstudio";
@@ -96,17 +95,11 @@ export default function AIAdminSettingsPage() {
   };
 
   if (isLoading) {
-    return (
-      <>
-        <Nav />
-        <div style={{ padding: 32 }}>Loading…</div>
-      </>
-    );
+    return <div style={{ padding: 32 }}>Loading…</div>;
   }
 
   return (
     <>
-      <Nav />
       <div style={{ padding: 32, maxWidth: 600, margin: "0 auto" }}>
         <h1 style={{ fontSize: 24, marginBottom: 32 }}>AI Service — Settings</h1>