bash-tasks page: live interval poll + spawn_blocking the dir scan (review)

Two fixes from review:
- Liveness: /api/state isn't polled while online (only during login), so
  hooking refreshBashTasks to it only populated on cold load. Bash tasks
  start + finish asynchronously between turns, so add a light ~4s interval
  to keep the tasks pill live; the /api/state-time call now just does the
  first-paint populate. Doc note corrected to match.
- Move the blocking dir scan + per-file reads in /api/bash-tasks off the
  async executor via tokio::task::spawn_blocking (damocles nit).

docs/web-ui/agent.md

@@ -63,9 +63,11 @@ through. Three flex columns:
     click opens the loose-ends flyout.
   - **Tasks pill** (`⚙ tasks · N`): hidden when empty; click opens the
     running-bash-tasks flyout (`GET /api/bash-tasks` — the in-flight
-    Pending/Running tasks from the in-container `bash-tasks/` dir). Polled
-    on the same `/api/state` cycle as loose-ends, since tasks complete
-    asynchronously between turns. Snapshot only (no SSE push yet).
+    Pending/Running tasks from the in-container `bash-tasks/` dir). Unlike
+    loose-ends (refreshed on turn_end), tasks start + finish asynchronously
+    between turns and `/api/state` isn't polled while online, so the pill
+    polls the endpoint on a light interval (≈4s). Snapshot only (no SSE push
+    yet).
   - **Overflow button** (`⋯`): always visible. Opens a frosted popover
     (`#overflow-menu`, positioned outside the header to escape any
     stacking context) with four management rows followed by a model