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

docker-compose.yml

@@ -10,6 +10,9 @@ services:
     environment:
       - LYRIA_API_KEY=${LYRIA_API_KEY}
       - LYRIA_DEFAULT_MODEL=${LYRIA_DEFAULT_MODEL:-lyria-3-clip-preview}
+      - KITHARA_MODE=${KITHARA_MODE:-single}
+      - KITHARA_ADMIN_USER=${KITHARA_ADMIN_USER:-admin}
+      - KITHARA_ADMIN_PASS=${KITHARA_ADMIN_PASS:-admin}
       - TZ=${TZ:-America/New_York}
     restart: unless-stopped
 
@@ -21,4 +24,4 @@ services:
       - "3100:3000"
     depends_on:
       - backend
-    restart: unless-stopped
+    restart: unless-stopped