feat(dashboard): highlight proposals pane + bell on new proposal

When a fresh proposal lands in the supervise queue, the
dashboard:

  1. Rings the terminal bell via `curses.beep()` so tmux's
     `monitor-bell` (or the terminal's own bell-on-activity)
     surfaces a notice in the dashboard pane even when the
     operator is focused on claude in the right pane.
  2. Bolds + green-attrs the `proposals:` pane label and
     suffixes it with `(new!)` so a glance at the dashboard
     screen catches the alert at a glance.

The highlight tracks the existing per-row green-highlight
window (`_NEW_PROPOSAL_HIGHLIGHT_SEC`). The bell only fires for
NEWLY arrived proposals after the first tick — pre-existing
queue entries on dashboard startup don't ring.

claude_bottle/cli/dashboard.py

@@ -1352,6 +1352,10 @@ def _main_loop(stdscr: "curses._CursesWindow") -> None:
         if manifest_cache[0] is None:
             manifest_cache[0] = Manifest.resolve(USER_CWD)
         return manifest_cache[0]
+    # First-tick guard: a brand-new dashboard finds any
+    # pre-existing queue entries on its first poll; those
+    # shouldn't ring the bell as if they just arrived.
+    saw_first_tick = False
     while True:
         pending = discover_pending()
         if selected >= len(pending):
@@ -1363,10 +1367,21 @@ def _main_loop(stdscr: "curses._CursesWindow") -> None:
 
         now = time.monotonic()
         live_ids = {qp.proposal.id for qp in pending}
+        # Detect proposals we've never seen before — used to ring
+        # the terminal bell (so tmux's monitor-bell or the
+        # terminal's own bell-on-activity surface a notice when
+        # the operator isn't looking at the dashboard pane).
+        newly_arrived = live_ids - first_seen.keys()
+        if saw_first_tick and newly_arrived:
+            try:
+                curses.beep()
+            except curses.error:
+                pass
         for proposal_id in live_ids:
             first_seen.setdefault(proposal_id, now)
         for stale_id in list(first_seen.keys() - live_ids):
             del first_seen[stale_id]
+        saw_first_tick = True
 
         _render(
             stdscr, pending, selected, status_line,
@@ -1541,10 +1556,22 @@ def _render(
 
     # ----- proposals pane (top) -----
     row = 2
+    # When any proposal is in the recent-arrival window (the
+    # individual rows are green-highlighted by the existing logic),
+    # also highlight the pane label so the alert is visible at a
+    # glance even when the operator is focused elsewhere.
+    proposals_have_recent = any(
+        _is_recent(qp.proposal.id, first_seen, now) for qp in pending
+    )
     proposals_label = "proposals:"
+    if proposals_have_recent:
+        proposals_label += "  (new!)"
     if proposals_focused:
         proposals_label += "  (focused)"
-    stdscr.addnstr(row, 0, proposals_label, w - 1, curses.A_DIM)
+    label_attr = curses.A_DIM
+    if proposals_have_recent:
+        label_attr = curses.A_BOLD | green_attr
+    stdscr.addnstr(row, 0, proposals_label, w - 1, label_attr)
     row += 1
     if not pending:
         stdscr.addnstr(