fix: add missing showNotice error handling and improve async UI feedback
- Add showNotice error notifications to unlock page async error branches
- Restore showNotice for YAML serialization errors in rules/groups/proxies editor
- Ensure all user-facing async errors are surfaced via showNotice
- Add fade-in animation to layout for smoother theme transition and reduce white screen
- Use requestIdleCallback/setTimeout for heavy UI state updates to avoid UI blocking
- Minor: remove window.showNotice usage, use direct import instead
