feat(mobile): single-pane stack + long-press tab menu + swipe-to-close

- Workspace: on mobile renders only panes[activePaneIdx] (rest skipped).
  When panes.length > 1, adds a horizontal pane-navigator strip built
  from SwipeablePaneTab above the active pane. URL state ?pane=<paneId>
  written by switchActivePane (user-initiated only) and read on URL
  change (back-button + deep-link). Bare URL resets activePaneIdx to 0.
- useLongPress: 500ms touchstart timer; on fire, dispatches a synthetic
  contextmenu event on target.closest('[data-tab-id]') so the existing
  Radix ContextMenuTrigger opens at the touch coordinates. Suppresses
  the synthetic click that follows touchend. Cancels on touchmove /
  touchend / touchcancel.
- ChatTabBar: each tab gets data-tab-id, touch handlers wired to
  useLongPress, and WebkitTouchCallout: 'none' to disable iOS Safari's
  text-selection callout.
- SwipeablePaneTab: tracks horizontal drag; bails if vertical delta
  exceeds 30px (so vertical scroll still works); past 60px on release
  fires onClose (removePane), else snaps back. Opacity fades 1->0.4
  approaching the threshold. Hand-rolled per spec.
- Pane drag-and-drop disabled on mobile (HTML5 drag is broken on touch
  anyway; mobile uses the navigator + swipe-to-close instead).

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

apps/web/src/components/ChatTabBar.tsx

@@ -8,6 +8,7 @@ import {
   ContextMenuSeparator,
   ContextMenuTrigger,
 } from '@/components/ui/context-menu';
+import { useLongPress } from '@/hooks/useLongPress';
 import { cn } from '@/lib/utils';
 
 interface Props {
@@ -40,6 +41,18 @@ export function ChatTabBar({
   const [renamingId, setRenamingId] = useState<string | null>(null);
   const [renameValue, setRenameValue] = useState('');
 
+  // Long-press: dispatch a synthetic contextmenu event on the tab so the
+  // existing Radix ContextMenuTrigger opens at the touch coordinates. Works
+  // because asChild composition makes the tab div the trigger element.
+  const longPress = useLongPress(({ clientX, clientY, target }) => {
+    if (!target || !(target instanceof Element)) return;
+    const tab = target.closest('[data-tab-id]') as HTMLElement | null;
+    if (!tab) return;
+    tab.dispatchEvent(
+      new MouseEvent('contextmenu', { bubbles: true, cancelable: true, clientX, clientY }),
+    );
+  });
+
   function startRename(chatId: string, currentName: string | null) {
     setRenamingId(chatId);
     setRenameValue(currentName ?? '');
@@ -63,7 +76,13 @@ export function ChatTabBar({
           <ContextMenu key={chat.id}>
             <ContextMenuTrigger asChild>
               <div
+                data-tab-id={chat.id}
                 onClick={() => onSwitchTab(tabIdx)}
+                onTouchStart={longPress.onTouchStart}
+                onTouchMove={longPress.onTouchMove}
+                onTouchEnd={longPress.onTouchEnd}
+                onTouchCancel={longPress.onTouchCancel}
+                style={{ WebkitTouchCallout: 'none' }}
                 className={cn(
                   'group flex items-center gap-1.5 px-3 py-1.5 text-xs border-r border-border cursor-default select-none shrink-0',
                   isActive