feat: improve agent status and thinking indicator UX

- Show agent activity label in chat header (Thinking…, Using bash…, etc.)
  instead of an anonymous pulsing dot
- Fix ThinkingProcess elapsed timer to tick live every second using
  useElapsedTimer hook instead of stale useMemo computation
- Show completed/failed subagents for 3s grace period with fade-out
  transition instead of instantly hiding them
- Transition activity labels to Completed state for 1.5s grace period
  on session idle instead of abrupt removal
- Eliminate message flash during thinking reclassification by folding
  trailing pending assistant messages into the thinking group
  optimistically when they follow existing thinking messages
- Stabilize ThinkingProcess isActive flag by checking for pending
  messages in the group rather than relying solely on array position
- Skip rendering empty pending messages inside ThinkingProcess steps

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/hooks/useElapsedTimer.ts

@@ -0,0 +1,39 @@
+import { useEffect, useState } from 'react';
+
+function formatElapsed(startMs: number): string {
+  const seconds = Math.max(0, Math.floor((Date.now() - startMs) / 1000));
+  if (seconds < 60) return `${seconds}s`;
+  const minutes = Math.floor(seconds / 60);
+  const remainder = seconds % 60;
+  return `${minutes}m ${remainder}s`;
+}
+
+/**
+ * Returns a live-ticking elapsed-time string while `active` is true.
+ * Freezes the display when `active` becomes false.
+ */
+export function useElapsedTimer(startedAt: string | undefined, active: boolean): string | undefined {
+  const startMs = startedAt ? new Date(startedAt).getTime() : undefined;
+
+  const [elapsed, setElapsed] = useState<string | undefined>(() => {
+    if (!startMs) return undefined;
+    return formatElapsed(startMs);
+  });
+
+  useEffect(() => {
+    if (!startMs) {
+      setElapsed(undefined);
+      return;
+    }
+
+    // Always sync to current value immediately
+    setElapsed(formatElapsed(startMs));
+
+    if (!active) return;
+
+    const id = setInterval(() => setElapsed(formatElapsed(startMs)), 1000);
+    return () => clearInterval(id);
+  }, [startMs, active]);
+
+  return elapsed;
+}