Theme scrollbars + global broadcast toggle

Two small QoL additions:

- styles.css: WebKit pseudo-element styling for .xterm-viewport
  scrollbars (8px wide, dark thumb #2a2a2a on transparent track,
  hover lighten). Matches the rest of the dark theme so the right
  edge of each terminal stops looking like default OS chrome.

- tree.ts: setAllBroadcast(root, on) helper that flips every leaf's
  broadcast flag to the given value, preserving object identity
  where nothing changed.

- App.tsx: titlebar 📡 button showing global broadcast state
  ("all off" / "all on" / "N/M"). Click toggles every pane between
  all-broadcasting and all-off. Orange when any panes are
  broadcasting; darker orange when partial.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.css

@@ -47,6 +47,15 @@
   color: #cce6ff;
   border-color: #2a5a8c;
 }
+.palette-btn.bcast-all.on {
+  background: #4a3010;
+  color: #f0c060;
+  border-color: #c98a1f;
+}
+.palette-btn.bcast-all.on.partial {
+  background: #2a2010;
+  color: #c98a1f;
+}
 .preset-btn {
   min-width: 28px;
   text-align: center;