feat(kanban): drag-to-delete trash zone + bulk delete for task cards

Salvages #28125 by @Jpalmer95. Adds:
- Drag-to-delete trash zone in the kanban dashboard
- Bulk delete endpoint with cascading delete_task cleanup
- Frontend updates (drag visual + drop handler)
- Confirmation prompt before delete

Resolved end-of-file test conflict by appending both halves.

tests/plugins/test_kanban_dashboard_plugin.py

@@ -485,6 +485,33 @@ def test_patch_status_running_rejected(client):
     assert statuses.get(t["id"]) != "running"
 
 
+# ---------------------------------------------------------------------------
+# DELETE /tasks/:id
+# ---------------------------------------------------------------------------
+
+def test_delete_task(client):
+    t = client.post("/api/plugins/kanban/tasks", json={"title": "to-delete"}).json()["task"]
+    r = client.delete(f"/api/plugins/kanban/tasks/{t['id']}")
+    assert r.status_code == 200
+    assert r.json()["deleted"] is True
+    assert r.json()["task_id"] == t["id"]
+
+    # Gone from board
+    board = client.get("/api/plugins/kanban/board").json()
+    all_ids = [tt["id"] for col in board["columns"] for tt in col["tasks"]]
+    assert t["id"] not in all_ids
+
+    # Gone from detail
+    r = client.get(f"/api/plugins/kanban/tasks/{t['id']}")
+    assert r.status_code == 404
+
+
+def test_delete_task_not_found(client):
+    r = client.delete("/api/plugins/kanban/tasks/t_nonexistent")
+    assert r.status_code == 404
+    assert "not found" in r.json()["detail"]
+
+
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