Implements the full SPA UI:
- App.tsx with BrowserRouter, ProtectedRoute, and all page routes
- Sidebar with NavLink active state and logout
- Login page with JWT auth flow
- Dashboard with parallel data fetching and status tiles
- MqttBrokers: CRUD with test-connection button and Topics link
- MqttTopics: discovery, filter, json_path inline edit, ignore
- PgSources: CRUD form with all required fields
- Datapoints: table with inline label/unit editing via onBlur
- Displays: layout dropdown, refresh interval, last-seen timestamp
- Layouts: left panel list + right panel element editor
- EpaperPreview: scaled e-paper simulation with live data support
- StatusBadge: color-coded status chip
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Sets up the frontend SPA foundation: Vite+React+TS project at frontend/,
configures Tailwind CSS v4 via @tailwindcss/vite, dev proxy to port 4000,
and implements the API client (JWT auth, auto-redirect on 401).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
