feat: support image lightbox viewer and file download (#151)

* feat: add lightbox overlay with nice mechanics

* ui tweak for top bar in stream view

* support locally downloading attachments

js/src/main.ts

@@ -454,6 +454,28 @@ ipcMain.handle('link:open-external', async (_event, url: string) => {
   await shell.openExternal(url);
 });
 
+// --- Attachment download ---
+// Triggers a native download with save-as dialog. Cross-origin safe — unlike
+// the web `<a download>` hack, which is ignored for cross-origin URLs.
+ipcMain.on(
+  'attachment:download',
+  (event, payload: { url: string; filename?: string }) => {
+    const win = BrowserWindow.fromWebContents(event.sender);
+    if (!win) return;
+    const { url, filename } = payload ?? {};
+    if (typeof url !== 'string') return;
+    if (!url.startsWith('http://') && !url.startsWith('https://')) return;
+
+    const dlSession = win.webContents.session;
+    const onWillDownload = (_e: Electron.Event, item: Electron.DownloadItem) => {
+      if (filename) item.setSaveDialogOptions({ defaultPath: filename });
+      dlSession.removeListener('will-download', onWillDownload);
+    };
+    dlSession.on('will-download', onWillDownload);
+    win.webContents.downloadURL(url);
+  },
+);
+
 // This method will be called when Electron has finished
 // initialization and is ready to create browser windows.
 // Some APIs can only be used after this event occurs.