feat: Phase 1 frontend - auth UI with KITHARA_MODE switch

- Auth API client (src/api/auth.ts): login, register, me, api-key endpoints
- AuthContext: React context managing auth state, mode detection, JWT tokens
- LoginPage: clean dark-themed login form with error handling
- RegisterPage: invite code + registration with password validation
- App.tsx: BrowserRouter + AuthProvider wrapper, mode-conditional routing
  - Single mode: straight to app, no auth required
  - Multi mode: redirect to /login, auth-gated access
- main.tsx: BrowserRouter + AuthProvider initialization
- react-router-dom added as dependency
- docker-compose.yml: KITHARA_MODE, KITHARA_ADMIN_USER, KITHARA_ADMIN_PASS env vars
- requirements.txt: pinned bcrypt==4.0.1 for passlib compatibility
- Tested: multi-mode admin bootstrap, JWT login, auth/me round-trip all confirmed

backend/requirements.txt

@@ -12,3 +12,4 @@ google-genai==1.0.0
 mutagen==1.47.0
 PyJWT==2.8.0
 passlib[bcrypt]==1.7.4
+bcrypt==4.0.1