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/lib/layout/orchestration.tsx

@@ -37,6 +37,11 @@ export interface Orchestration {
   beginHeaderDrag: (leafId: NodeId) => void;
   setHeaderDragOver: (leafId: NodeId | null) => void;
   endHeaderDrag: (commitSwap: boolean) => void;
+
+  // Per-leaf idle reporting. LeafPanes call reportLeafIdle when their
+  // own quiet-state crosses the threshold; App aggregates so the titlebar
+  // can show an "N idle" count without spamming toast notifications.
+  reportLeafIdle: (leafId: NodeId, idle: boolean) => void;
 }
 
 const OrchestrationContext = createContext<Orchestration | null>(null);