agent web UI: bulk mark-done for the todos flyout

The per-agent web UI todos flyout (loose-ends v2) had no mark-done
affordance at all — dismissing a todo was only possible via the
cancel_loose_end MCP tool, one id at a time. Add a checkbox per row,
a select-all/select-none/mark-done bulk row, and a new
POST /api/todos/mark-done handler that loops the existing single-id
MarkTodoDone request over the in-agent socket (no new wire request
type needed — the todos list is small, so N same-host round-trips is
cheap).

Fixes #2917

hive-agent/src/web_ui/mod.rs

@@ -116,6 +116,7 @@ pub async fn serve(
         .route("/api/new-session", post(actions::post_new_session))
         .route("/api/logout", post(auth::post_logout))
         .route("/api/todos", get(stats::api_todos))
+        .route("/api/todos/mark-done", post(actions::post_mark_todos_done))
         .route("/api/stats", get(stats::api_stats))
         .route("/screen/ws", get(screen::screen_ws))
         .route("/icon", get(screen::serve_icon));