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/stores/playback-store.ts

@@ -0,0 +1,18 @@
+import { create } from "zustand";
+
+/**
+ * Overlays that should pause stream playback (e.g. attachment lightbox) call
+ * `suspend()` on mount and `release()` on unmount. Stream playback is suspended
+ * whenever `suspendCount > 0`.
+ */
+interface PlaybackState {
+  suspendCount: number;
+  suspend: () => void;
+  release: () => void;
+}
+
+export const usePlaybackStore = create<PlaybackState>((set) => ({
+  suspendCount: 0,
+  suspend: () => set((s) => ({ suspendCount: s.suspendCount + 1 })),
+  release: () => set((s) => ({ suspendCount: Math.max(0, s.suspendCount - 1) })),
+}));