修复多会话任务栏抖动、会话跳回与停止失效 (#264)

* fix(web): stabilize active task ordering

* fix(chat): preserve navigation during conversation startup

* fix(tasks): guarantee hard cancellation

* fix(chat): bind navigation and hard stop targets

* fix(chat): prevent replay from reclaiming navigation

internal/handler/task_manager_order_test.go

@@ -0,0 +1,31 @@
+package handler
+
+import (
+	"testing"
+	"time"
+)
+
+func TestGetActiveTasksUsesStableCreationOrder(t *testing.T) {
+	m := NewAgentTaskManager()
+	started := time.Date(2026, 8, 19, 10, 0, 0, 0, time.UTC)
+	m.mu.Lock()
+	m.tasks = map[string]*AgentTask{
+		"conversation-z":    {ConversationID: "conversation-z", StartedAt: started, Status: "running"},
+		"conversation-late": {ConversationID: "conversation-late", StartedAt: started.Add(time.Minute), Status: "running"},
+		"conversation-a":    {ConversationID: "conversation-a", StartedAt: started, Status: "running"},
+	}
+	m.mu.Unlock()
+
+	want := []string{"conversation-a", "conversation-z", "conversation-late"}
+	for attempt := 0; attempt < 20; attempt++ {
+		gotTasks := m.GetActiveTasks()
+		if len(gotTasks) != len(want) {
+			t.Fatalf("GetActiveTasks() length = %d, want %d", len(gotTasks), len(want))
+		}
+		for i, task := range gotTasks {
+			if task.ConversationID != want[i] {
+				t.Fatalf("attempt %d order[%d] = %q, want %q", attempt, i, task.ConversationID, want[i])
+			}
+		}
+	}
+}