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>

tests/renderer/sessionActivity.test.ts

@@ -13,6 +13,8 @@ import {
   isAgentActivityCompleted,
   pruneSessionActivities,
   pruneSessionRequestUsage,
+  purgeCompletedActivity,
+  summarizeSessionActivity,
   type SessionActivityMap,
   type SessionRequestUsageMap,
 } from '@renderer/lib/sessionActivity';
@@ -202,6 +204,7 @@ describe('session activity helpers', () => {
       },
     };
 
+    // Active agents transition to 'completed' on idle (grace period before purge)
     expect(
       applySessionEventActivity(current, {
         sessionId: 'session-1',
@@ -216,6 +219,11 @@ describe('session activity helpers', () => {
           agentName: 'Architect',
           activityType: 'completed',
         },
+        reviewer: {
+          agentId: 'reviewer',
+          agentName: 'Reviewer',
+          activityType: 'completed',
+        },
       },
     });
   });
@@ -236,6 +244,7 @@ describe('session activity helpers', () => {
       },
     };
 
+    // Active agents transition to 'completed' on cancel (grace period before purge)
     expect(
       applySessionEventActivity(current, {
         sessionId: 'session-1',
@@ -265,6 +274,11 @@ describe('session activity helpers', () => {
           agentName: 'Architect',
           activityType: 'completed',
         },
+        reviewer: {
+          agentId: 'reviewer',
+          agentName: 'Reviewer',
+          activityType: 'completed',
+        },
       },
     });
   });
@@ -385,6 +399,92 @@ describe('session activity helpers', () => {
       'session-2': current['session-2'],
     });
   });
+
+  test('purgeCompletedActivity removes completed entries after grace period', () => {
+    const current: SessionActivityMap = {
+      'session-1': {
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'completed',
+        },
+        reviewer: {
+          agentId: 'reviewer',
+          agentName: 'Reviewer',
+          activityType: 'completed',
+        },
+      },
+    };
+
+    const result = purgeCompletedActivity(current, 'session-1');
+    expect(result['session-1']).toBeUndefined();
+  });
+
+  test('purgeCompletedActivity is a no-op when nothing is completed', () => {
+    const current: SessionActivityMap = {
+      'session-1': {
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'thinking',
+        },
+      },
+    };
+    expect(purgeCompletedActivity(current, 'session-1')).toBe(current);
+  });
+
+  test('summarizeSessionActivity returns the most relevant label', () => {
+    expect(summarizeSessionActivity(undefined)).toBeUndefined();
+    expect(summarizeSessionActivity({})).toBeUndefined();
+
+    expect(
+      summarizeSessionActivity({
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'thinking',
+        },
+      }),
+    ).toBe('Thinking…');
+
+    expect(
+      summarizeSessionActivity({
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'tool-calling',
+          toolName: 'bash',
+        },
+      }),
+    ).toBe('Using bash…');
+
+    // Thinking takes priority over tool-calling
+    expect(
+      summarizeSessionActivity({
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'thinking',
+        },
+        reviewer: {
+          agentId: 'reviewer',
+          agentName: 'Reviewer',
+          activityType: 'tool-calling',
+          toolName: 'read_file',
+        },
+      }),
+    ).toBe('Thinking…');
+
+    expect(
+      summarizeSessionActivity({
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'completed',
+        },
+      }),
+    ).toBe('Completed');
+  });
 });
 
 describe('assistant usage accumulator', () => {