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/electron.d.ts

@@ -44,6 +44,9 @@ declare global {
       fetchMetadata: (url: string) => Promise<LinkMetadata | null>;
       openExternal: (url: string) => Promise<void>;
     };
+    electronAttachment: {
+      download: (url: string, filename?: string) => void;
+    };
     electronApp: {
       setDockBadge: (count: number) => void;
     };