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/tenancy/services/admin_platform_service.py

@@ -23,6 +23,7 @@ from app.modules.tenancy.exceptions import (
 from app.modules.tenancy.models import AdminPlatform
 from app.modules.tenancy.models import Platform
 from app.modules.tenancy.models import User
+from models.schema.auth import UserContext
 
 logger = logging.getLogger(__name__)
 
@@ -223,14 +224,14 @@ class AdminPlatformService:
 
     def validate_admin_platform_access(
         self,
-        user: User,
+        user: User | UserContext,
         platform_id: int,
     ) -> None:
         """
         Validate that an admin has access to a platform.
 
         Args:
-            user: User object
+            user: User model or UserContext schema (both have can_access_platform)
             platform_id: Platform ID to check
 
         Raises: