fix: settings pane close affordance + sidebar toggle

The v1.9 settings pane had no way to dismiss once opened. ChatTabBar
(which owns the per-pane close X for chat panes) is skipped for
settings panes, and the pane header itself only rendered the maximize
toggle (desktop-only). Mobile users had zero controls beyond the
section tabs.

Add three close paths:

- X button in SettingsPane header, visible on mobile + desktop, sits
  next to the maximize toggle. Tap-target sized per the v1.6 mobile
  convention (max-md:min-h-[44px]).
- Esc when the settings pane is the active pane and no input/textarea/
  dialog has focus. Maximize-restore still wins when maximized.
- Sidebar Settings button is now a strict toggle: opens on first click,
  closes on second. Renamed openOrFocusSettingsPane →
  toggleSettingsPane in the panes hook.

Edge case: removing the settings pane when it's the only pane left
falls back to an empty pane to preserve the "always one pane"
invariant. In normal flow this is unreachable (the toggle only
appends), but defensive against future entry points.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/useSidebar.ts

@@ -152,8 +152,8 @@ function applyEvent(prev: SidebarResponse, event: import('./sessionEvents').Sess
       // Consumed by Workspace; sidebar has no business with pane state.
       return prev;
     case 'open_settings_pane':
-      // v1.9: consumed by Session.tsx (calls openOrFocusSettingsPane on its
-      // panesHook). Sidebar data is untouched.
+      // Consumed by Session.tsx (calls toggleSettingsPane on its panesHook).
+      // Sidebar data is untouched.
       return prev;
     case 'session_archived': {
       let changed = false;