fix: resolve lint errors from ESLint upgrade

- github-api.js: attach cause to re-thrown timeout error (preserve-caught-error)
- popup.js: remove useless initial value on `message` declaration
- popup.js: remove redundant `className` reassignment in else branch (no-useless-assignment)

Author: euxx

src/lib/github-api.js

@@ -46,7 +46,7 @@ async function fetchWithTimeout(url, options = {}, timeout = API_TIMEOUTS.DEFAUL
     return response;
   } catch (error) {
     if (error.name === 'AbortError') {
-      throw new Error('Request timeout');
+      throw new Error('Request timeout', { cause: error });
     }
     throw error;
   } finally {

src/popup/popup.js

@@ -618,7 +618,7 @@ async function refresh() {
     const cachedNotifications = await storage.getNotifications();
     renderNotifications(cachedNotifications, true); // Re-sort even on error
 
-    let message = '';
+    let message;
     let className = 'error-message';
 
     if (!navigator.onLine || error.message?.includes('NetworkError') || error.message?.includes('Failed to fetch')) {
@@ -632,7 +632,6 @@ async function refresh() {
       className = 'warning-message';
     } else {
       message = `❌ Error: ${error.message || 'Failed to refresh'}`;
-      className = 'error-message';
     }
 
     // Show error/warning message