[codex] Add app URL routes

[colinthebomb1]

Summary

• Replace app-shell view-only navigation with real browser paths for landing, login, dashboard, profile, and challenge screens.
• Add challenge subroutes: /challenges/:id, /challenges/:id/attack, and /challenges/:id/defend.
• Wire Back to Overview through route navigation so attack/defend modes do not immediately reopen.
• Keep attack/defend deep links behind the existing reading-summary gate.
• Return a clean 503 when Atlas is reconnecting during auth lookups instead of dumping raw PyMongo tracebacks; keep /api/auth/me usable if streak lookup hiccups.

Validation

• git diff --check -- Frontend/src/app/App.tsx Frontend/src/app/components/ChallengePlay.tsx backend/app/routers/auth.py
• cd backend && ./.venv/bin/pytest -q -s tests/test_auth_api.py tests/test_api.py::test_me_returns_anonymous_without_session

Note: frontend build remains blocked by the existing local Rollup optional dependency issue: missing @rollup/rollup-win32-x64-msvc.