Add configurable default theme

[vkhurana]

Summary

• add ui.default_theme to config.yaml
• expose the configured theme to the frontend while preserving saved browser preferences
• apply the configured default on the login page
• validate theme IDs and fall back to light when the configured theme is unavailable
• document the new option

Verification

• python3 -m compileall -q backend mcp_server
• node --check frontend/app.js
• git diff --check
• parsed config.yaml and verified the configured theme file exists