Show open Vikunja task count as badge on To-Do sidebar link

Fetched async via /api/todos/count so it doesn't block page render.
Badge hidden when count is zero.

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

app/routers/todos.py

@@ -4,6 +4,12 @@ from app.vikunja import create_task, delete_task, get_tasks, update_task
 router = APIRouter(prefix="/todos", tags=["todos"])
 
 
+@router.get("/count")
+def count_todos():
+    tasks = get_tasks()
+    return {"open": sum(1 for t in tasks if not t.get("done"))}
+
+
 @router.get("/")
 def list_todos():
     return get_tasks()