Replace idle toasts with pane border + titlebar badge

Old behaviour: every pane fired orch.notify("X is idle") after 5s of
silence, stacking up to N toasts that took ages to dismiss.

New behaviour:
- LeafPane tracks its own isIdle state locally and reports up via
  orch.reportLeafIdle(leafId, idle).
- App aggregates into a Set<NodeId> and renders "N idle" in red after
  the "N panes" count in the titlebar (hidden when zero).
- The pane itself gets a red border (.leaf.idle) — but active and
  broadcasting borders still take precedence, so the focus indicator
  isn't masked by idle status.
- The pane's "alive" status text in the toolbar swaps to red "idle"
  while it's quiet (reverts to "alive" the moment output arrives).
- Idle clears immediately on the next byte of output (no 1-second lag)
  AND when the pane unmounts (cleanup effect).

No more flood of toasts.

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

src/App.tsx

@@ -239,6 +239,19 @@ export default function App() {
     setNotifications((ns) => ns.filter((n) => n.id !== id));
   }, []);
 
+  // ---- per-pane idle aggregation (replaces toast spam) --------------------
+  const [idleLeafIds, setIdleLeafIds] = useState<Set<NodeId>>(() => new Set());
+  const reportLeafIdle = useCallback((leafId: NodeId, idle: boolean) => {
+    setIdleLeafIds((prev) => {
+      if (idle && prev.has(leafId)) return prev;
+      if (!idle && !prev.has(leafId)) return prev;
+      const next = new Set(prev);
+      if (idle) next.add(leafId);
+      else next.delete(leafId);
+      return next;
+    });
+  }, []);
+
   // ---- header-drag swap ---------------------------------------------------
   const [dragSourceId, setDragSourceId] = useState<NodeId | null>(null);
   const [dragOverId, setDragOverId] = useState<NodeId | null>(null);
@@ -283,6 +296,7 @@ export default function App() {
       beginHeaderDrag,
       setHeaderDragOver,
       endHeaderDrag,
+      reportLeafIdle,
     }),
     [
       activeLeafId,
@@ -301,6 +315,7 @@ export default function App() {
       beginHeaderDrag,
       setHeaderDragOver,
       endHeaderDrag,
+      reportLeafIdle,
     ],
   );
 
@@ -443,6 +458,12 @@ export default function App() {
 
         <span className="layout-info">
           {leafCount(tree)} pane{leafCount(tree) === 1 ? "" : "s"}
+          {idleLeafIds.size > 0 && (
+            <span className="idle-info" title="Panes that haven't produced output recently">
+              {" · "}
+              {idleLeafIds.size} idle
+            </span>
+          )}
         </span>
       </header>