fix: platform admin authentication and UserContext completeness

Issues fixed:
- Platform selection returned LoginResponse requiring user timestamps,
  but UserContext doesn't have created_at/updated_at. Created dedicated
  PlatformSelectResponse that returns only token and platform info.

- UserContext was missing platform context fields (token_platform_id,
  token_platform_code). JWT token included them but they weren't
  extracted into UserContext, causing fallback warnings.

- admin_menu_config.py accessed admin_platforms (SQLAlchemy relationship)
  on UserContext (Pydantic schema). Changed to use accessible_platform_ids.

- Static file mount order in main.py caused 404 for locale files.
  More specific paths (/static/modules/X/locales) must be mounted
  before less specific paths (/static/modules/X).

Changes:
- models/schema/auth.py: Add PlatformSelectResponse, token_platform_id,
  token_platform_code, can_access_platform(), get_accessible_platform_ids()
- admin_auth.py: Use PlatformSelectResponse for select-platform endpoint
- admin_platform_service.py: Accept User | UserContext in validation
- admin_menu_config.py: Use accessible_platform_ids instead of admin_platforms
- main.py: Mount locales before static for correct path priority

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/core/routes/api/admin_menu_config.py

@@ -431,14 +431,14 @@ async def get_rendered_admin_menu(
     else:
         # Platform admin: use platform-level config
         # Get the selected platform from the JWT token
-        platform_id = getattr(current_user, "token_platform_id", None)
+        platform_id = current_user.token_platform_id
 
-        # Fallback to first platform if no platform in token (shouldn't happen)
-        if platform_id is None and current_user.admin_platforms:
-            platform_id = current_user.admin_platforms[0].id
+        # Fallback to first accessible platform if no platform in token (shouldn't happen)
+        if platform_id is None and current_user.accessible_platform_ids:
+            platform_id = current_user.accessible_platform_ids[0]
             logger.warning(
                 f"[MENU_CONFIG] No platform_id in token for {current_user.email}, "
-                f"falling back to first platform: {platform_id}"
+                f"falling back to first accessible platform: {platform_id}"
             )
 
         menu = menu_service.get_menu_for_rendering(