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/dependencies.py

@@ -42,3 +42,17 @@ async def get_current_user(
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not found")
 
     return user
+
+
+async def get_admin_user(
+    credentials: Optional[HTTPAuthorizationCredentials] = Depends(bearer_scheme),
+) -> dict:
+    if not credentials:
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Not authenticated")
+    try:
+        payload = decode_token(credentials.credentials)
+    except ValueError:
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid token")
+    if payload.get("type") != "access" or payload.get("role") != "admin":
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Admin access required")
+    return payload