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.tsx

@@ -21,6 +21,7 @@ import {
   changeDistro,
   changeLabel,
   toggleBroadcast as toggleBroadcastInTree,
+  setAllBroadcast,
   serialize,
   deserialize,
   presetSingle,
@@ -282,6 +283,29 @@ export default function App() {
     [paletteOpen, tree],
   );
 
+  // ---- global broadcast state (derived from tree) -------------------------
+  const broadcastStats = useMemo(() => {
+    let on = 0;
+    let total = 0;
+    for (const leaf of walkLeaves(tree)) {
+      total++;
+      if (leaf.broadcast) on++;
+    }
+    return { on, total };
+  }, [tree]);
+
+  const toggleBroadcastAll = useCallback(() => {
+    // If any pane is broadcasting, turn them all off. Otherwise turn them all on.
+    setTree((t) => setAllBroadcast(t, broadcastStats.on === 0));
+  }, [broadcastStats.on]);
+
+  const broadcastBtnLabel =
+    broadcastStats.on === 0
+      ? "📡 all off"
+      : broadcastStats.on === broadcastStats.total
+        ? "📡 all on"
+        : `📡 ${broadcastStats.on}/${broadcastStats.total}`;
+
   const onPalettePick = useCallback((leafId: string) => {
     setActiveLeafId(leafId);
     setPaletteOpen(false);
@@ -321,6 +345,20 @@ export default function App() {
           <button className="preset-btn" title="2 × 2 grid" onClick={() => applyPreset(presetTwoByTwo)}>2×2</button>
         </span>
 
+        <button
+          className={`palette-btn bcast-all${broadcastStats.on > 0 ? " on" : ""}${broadcastStats.on > 0 && broadcastStats.on < broadcastStats.total ? " partial" : ""}`}
+          onClick={toggleBroadcastAll}
+          title={
+            broadcastStats.on === 0
+              ? "Click to broadcast input to ALL panes"
+              : broadcastStats.on === broadcastStats.total
+                ? "All panes broadcasting — click to disable"
+                : `${broadcastStats.on} of ${broadcastStats.total} panes broadcasting — click to disable all`
+          }
+        >
+          {broadcastBtnLabel}
+        </button>
+
         <button
           className="palette-btn"
           onClick={() => setPaletteOpen(true)}