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.css

@@ -70,6 +70,9 @@
   color: #777;
   font-size: 11px;
 }
+.layout-info .idle-info {
+  color: #d96060;
+}
 
 .pane-wrap {
   flex: 1 1 auto;