refactor: improve error handling and code formatting

- Enhanced error messages in Rust backend to include file paths
- Improved provider switching error handling with detailed messages
- Added MCP button placeholder in UI (functionality TODO)
- Applied code formatting across frontend components
- Extended error notification duration to 6s for better readability

src/i18n/index.ts

@@ -20,7 +20,8 @@ const getInitialLanguage = (): "zh" | "en" => {
 
   const navigatorLang =
     typeof navigator !== "undefined"
-      ? navigator.language?.toLowerCase() ?? navigator.languages?.[0]?.toLowerCase()
+      ? (navigator.language?.toLowerCase() ??
+        navigator.languages?.[0]?.toLowerCase())
       : undefined;
 
   if (navigatorLang?.startsWith("zh")) {