Add cache clearing documentation for electron-updater

- Introduced CLEAR-UPDATE-CACHE.md to guide users on clearing the electron-updater cache across macOS, Windows, and Linux.
- Added programmatic method for cache clearing in JavaScript.
- Enhanced update handling in main.js and preload.js to support new update events.
- Updated GUI styles for download buttons and progress indicators in update.js and style.css.

backend/appUpdater.js

@@ -43,9 +43,17 @@ class AppUpdater {
       if (this.mainWindow && !this.mainWindow.isDestroyed()) {
         this.mainWindow.webContents.send('update-available', {
           version: info.version,
+          newVersion: info.version,
+          currentVersion: app.getVersion(),
           releaseName: info.releaseName,
           releaseNotes: info.releaseNotes
         });
+        // Also send to the old popup handler for compatibility
+        this.mainWindow.webContents.send('show-update-popup', {
+          currentVersion: app.getVersion(),
+          newVersion: info.version,
+          version: info.version
+        });
       }
     });