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/styles.css

@@ -37,3 +37,29 @@ body {
 
 .xterm { height: 100%; }
 .xterm-viewport { background: #0c0c0c !important; }
+
+/* Themed scrollbars — Chromium pseudo-elements (WebView2 supports these). */
+.xterm-viewport::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+.xterm-viewport::-webkit-scrollbar-track {
+  background: transparent;
+}
+.xterm-viewport::-webkit-scrollbar-thumb {
+  background: #2a2a2a;
+  border-radius: 4px;
+  border: 1px solid #1a1a1a;
+}
+.xterm-viewport::-webkit-scrollbar-thumb:hover {
+  background: #3a3a3a;
+}
+.xterm-viewport::-webkit-scrollbar-corner {
+  background: transparent;
+}
+/* Firefox fallback (and the new spec) — not strictly needed in WebView2
+   but free-and-correct. */
+.xterm-viewport {
+  scrollbar-width: thin;
+  scrollbar-color: #2a2a2a transparent;
+}