Fix M4 reactivity bugs: active border, Ctrl+K, diagnostics

- Drill activeLeafId as a separate prop through Pane -> SplitNode ->
  LeafPane instead of bundling it into the \$derived ops object.
  Passing activeLeafId via ops caused subsequent focus changes to
  not propagate to children (LeafPane's active = \$derived(...) wasn't
  re-evaluating when ops's identity changed). Drilling sidesteps any
  prop-as-derived-object reactivity quirks.
- Ctrl+K listener now uses capture phase so it wins over xterm.js's
  keydown handler inside the focused terminal.
- Bump active/broadcasting borders to 2px and brighter colors so the
  visual change is unmissable.
- Add a 🔔 test-toast button in the titlebar to verify the
  notification pipeline independently of idle detection.
- Sprinkle console.log diagnostics through the active/broadcast/
  idle/notify flows so we can pinpoint any remaining issues from
  devtools next time something looks off.

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

src/lib/layout/Pane.svelte

@@ -7,16 +7,18 @@
   let {
     node,
     ops,
+    activeLeafId,
   }: {
     node: TreeNode;
     ops: PaneOps;
+    activeLeafId: string | null;
   } = $props();
 </script>
 
 {#if node.kind === "split"}
-  <SplitNode {node} {ops} />
+  <SplitNode {node} {ops} {activeLeafId} />
 {:else}
   {#key node.id}
-    <LeafPane leaf={node} {ops} />
+    <LeafPane leaf={node} {ops} {activeLeafId} />
   {/key}
 {/if}