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/preload.ts

@@ -66,6 +66,11 @@ contextBridge.exposeInMainWorld('electronLink', {
   openExternal: (url: string) => ipcRenderer.invoke('link:open-external', url),
 });
 
+contextBridge.exposeInMainWorld('electronAttachment', {
+  download: (url: string, filename?: string) =>
+    ipcRenderer.send('attachment:download', { url, filename }),
+});
+
 contextBridge.exposeInMainWorld('electronApp', {
   setDockBadge: (count: number) => ipcRenderer.send('app:set-dock-badge', count),
 });