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/useWorkspacePanes.ts

@@ -73,10 +73,10 @@ export interface UseWorkspacePanesResult {
   closeAllTabs: (paneIdx: number) => void;
   showLandingPage: (paneIdx: number) => void;
   addSplitPane: (kind: 'chat' | 'terminal' | 'agent') => void;
-  // v1.9: idempotent open-or-focus for the settings pane singleton. Appends
-  // a new settings pane if none exists, otherwise just focuses the existing
-  // one. Always succeeds — settings panes don't count toward MAX_PANES.
-  openOrFocusSettingsPane: () => void;
+  // Open-on-first-click, close-on-second-click. Singleton — settings panes
+  // don't count toward MAX_PANES. Closing the only remaining pane (edge case)
+  // falls back to an empty pane to preserve the "always one pane" invariant.
+  toggleSettingsPane: () => void;
   removePane: (idx: number) => void;
   removeChatFromPanes: (chatId: string) => void;
   initializeFirstChatIfEmpty: (chatId: string) => void;
@@ -254,22 +254,35 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     });
   }, []);
 
-  const openOrFocusSettingsPane = useCallback(() => {
+  const toggleSettingsPane = useCallback(() => {
     setPanes((prev) => {
       const existingIdx = prev.findIndex((p) => p.kind === 'settings');
-      if (existingIdx >= 0) {
-        setActivePaneIdx(existingIdx);
-        return prev;
+      if (existingIdx < 0) {
+        const next = [...prev, settingsPane()];
+        setActivePaneIdx(next.length - 1);
+        return next;
       }
-      const next = [...prev, settingsPane()];
-      setActivePaneIdx(next.length - 1);
+      if (prev.length <= 1) {
+        setActivePaneIdx(0);
+        return [emptyPane()];
+      }
+      const next = prev.filter((_, i) => i !== existingIdx);
+      setActivePaneIdx((ai) => Math.min(ai, next.length - 1));
       return next;
     });
   }, []);
 
   const removePane = useCallback((idx: number) => {
     setPanes((prev) => {
-      if (prev.length <= 1) return prev;
+      if (prev.length <= 1) {
+        // Settings is the only kind that can be the last pane and still need
+        // closing (X / Esc / sidebar toggle). Fall back to empty.
+        if (prev[idx]?.kind === 'settings') {
+          setActivePaneIdx(0);
+          return [emptyPane()];
+        }
+        return prev;
+      }
       const next = prev.filter((_, i) => i !== idx);
       setActivePaneIdx((ai) => Math.min(ai, next.length - 1));
       return next;
@@ -359,7 +372,6 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     setActivePaneIdx,
     activePaneIdxRef,
     openChatInPane,
-    openOrFocusSettingsPane,
     switchTab,
     removeTab,
     closeOtherTabs,
@@ -367,6 +379,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     closeAllTabs,
     showLandingPage,
     addSplitPane,
+    toggleSettingsPane,
     removePane,
     removeChatFromPanes,
     initializeFirstChatIfEmpty,