Add Super Admin panel with user impersonation

- New /super-admin/login and /super-admin routes with separate auth
- Super admin can view all registered accounts and impersonate any user
- Impersonation banner shows at top of screen with exit button
- ADMIN_USERNAME and ADMIN_PASSWORD config added to .env and docker-compose.yml
- Fixed auth store: export setToken, clearToken, and setUser so they are
  accessible from superAdmin store
- Updated README with super admin feature, new env vars, and setup notes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/auth/jwt.py

@@ -26,6 +26,13 @@ def create_access_token(data: dict[str, Any]) -> str:
     return jwt.encode(payload, settings.secret_key, algorithm=settings.algorithm)
 
 
+def create_admin_token(data: dict[str, Any]) -> str:
+    payload = data.copy()
+    expire = datetime.now(timezone.utc) + timedelta(hours=8)
+    payload.update({"exp": expire, "type": "access", "role": "admin"})
+    return jwt.encode(payload, settings.secret_key, algorithm=settings.algorithm)
+
+
 def create_refresh_token(data: dict[str, Any]) -> str:
     payload = data.copy()
     expire = datetime.now(timezone.utc) + timedelta(days=settings.refresh_token_expire_days)