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/tailwind.config.ts

@@ -0,0 +1,30 @@
+import type { Config } from "tailwindcss";
+
+export default {
+  darkMode: ["class"],
+  content: ["./index.html", "./src/**/*.{ts,tsx}"],
+  theme: {
+    extend: {
+      colors: {
+        // Brand / interactive
+        primary: "rgb(var(--color-primary) / <alpha-value>)",
+        "primary-hover": "rgb(var(--color-primary-hover) / <alpha-value>)",
+        accent: "rgb(var(--color-accent) / <alpha-value>)",
+        "accent-hover": "rgb(var(--color-accent-hover) / <alpha-value>)",
+        // Layout
+        background: "rgb(var(--color-background) / <alpha-value>)",
+        surface: "rgb(var(--color-surface) / <alpha-value>)",
+        border: "rgb(var(--color-border) / <alpha-value>)",
+        // Text
+        foreground: "rgb(var(--color-text-primary) / <alpha-value>)",
+        muted: "rgb(var(--color-text-muted) / <alpha-value>)",
+      },
+      borderRadius: {
+        lg: "var(--radius)",
+        md: "calc(var(--radius) - 2px)",
+        sm: "calc(var(--radius) - 4px)",
+      },
+    },
+  },
+  plugins: [],
+} satisfies Config;