feat: interactive Vikunja tasks — checkbox toggles done, click opens task

- Added POST /api/tasks/toggle endpoint to mark tasks as done/undone
- Added toggle_task_done() in vikunja_service (POST /tasks/{id})
- Cache invalidated after toggle for immediate refresh
- Checkbox click toggles done state with visual feedback
- Click on task row opens Vikunja in new tab (/tasks/{id})
- ExternalLink icon appears on hover as affordance

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/services/vikunja_service.py

@@ -171,6 +171,46 @@ async def fetch_tasks(base_url: str, token: str) -> Dict[str, Any]:
     return result
 
 
+async def toggle_task_done(
+    base_url: str,
+    token: str,
+    task_id: int,
+    done: bool,
+) -> Dict[str, Any]:
+    """Toggle the done state of a single Vikunja task.
+
+    Args:
+        base_url: Vikunja instance base URL.
+        token: API token for authentication.
+        task_id: The task ID to update.
+        done: New done state.
+
+    Returns:
+        Dictionary with ``ok`` and optionally ``error``.
+    """
+    if not base_url or not token:
+        return {"ok": False, "error": "Missing Vikunja base URL or token"}
+
+    clean_url = base_url.rstrip("/")
+    headers = {
+        "Authorization": f"Bearer {token}",
+        "Content-Type": "application/json",
+    }
+
+    try:
+        async with httpx.AsyncClient(timeout=10, headers=headers) as client:
+            resp = await client.post(
+                f"{clean_url}/tasks/{task_id}",
+                json={"done": done},
+            )
+            resp.raise_for_status()
+            return {"ok": True, "task": {"id": task_id, "done": done}}
+    except httpx.HTTPStatusError as exc:
+        return {"ok": False, "error": f"HTTP {exc.response.status_code}"}
+    except Exception as exc:
+        return {"ok": False, "error": str(exc)}
+
+
 async def fetch_single_project(
     base_url: str,
     token: str,