fix(auth): preserve next on login + default home to realtime map

[Cheewye]

What changed

• Home route (/) now redirects to /map?start=realtime (removes forced mode=preparacion).
• Protected routes now redirect unauthenticated users to /login?next=<current path + search + hash>.
• After successful login, the app navigates back to next (only if it starts with /), otherwise falls back to /map?start=realtime (prevents open redirect).

Why

• Avoid surprising default state (mode=preparacion) as a global entry URL.
• Preserve user intent when auth is required (standard next pattern).
• Improve security by blocking external/open redirects.

Files changed (frontend only)

• frontend/src/App.tsx
• frontend/src/components/auth/ProtectedRoute.tsx
• frontend/src/pages/LoginPage.tsx

Verification

• npm -C frontend run build ✅ PASS
• Manual:
  • Visiting / redirects to /map?start=realtime
  • Visiting /map?start=realtime while logged out redirects to /login?next=/map?start=realtime
  • After login, returns to next (or safe fallback)