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/hooks/useLongPress.ts

@@ -0,0 +1,75 @@
+import { useCallback, useRef } from 'react';
+import type { TouchEvent } from 'react';
+
+interface LongPressHandlers {
+  onTouchStart: (e: TouchEvent) => void;
+  onTouchMove: (e: TouchEvent) => void;
+  onTouchEnd: (e: TouchEvent) => void;
+  onTouchCancel: (e: TouchEvent) => void;
+}
+
+interface Options {
+  ms?: number;
+  // Suppress the synthetic click that follows touchend when long-press fired.
+  suppressClickOnFire?: boolean;
+}
+
+// Hand-rolled long-press detector. Starts a timer on touchstart; cancels on
+// touchmove or early touchend; fires the callback on timer expiry. Caller is
+// expected to suppress text-selection callout via CSS (-webkit-touch-callout).
+export function useLongPress(
+  callback: (touch: { clientX: number; clientY: number; target: EventTarget | null }) => void,
+  { ms = 500, suppressClickOnFire = true }: Options = {},
+): LongPressHandlers {
+  const timerRef = useRef<number | null>(null);
+  const firedRef = useRef(false);
+
+  const clear = useCallback(() => {
+    if (timerRef.current !== null) {
+      window.clearTimeout(timerRef.current);
+      timerRef.current = null;
+    }
+  }, []);
+
+  const onTouchStart = useCallback(
+    (e: TouchEvent) => {
+      firedRef.current = false;
+      const touch = e.touches[0];
+      if (!touch) return;
+      const x = touch.clientX;
+      const y = touch.clientY;
+      const target = e.target;
+      clear();
+      timerRef.current = window.setTimeout(() => {
+        firedRef.current = true;
+        callback({ clientX: x, clientY: y, target });
+      }, ms);
+    },
+    [callback, ms, clear],
+  );
+
+  const onTouchMove = useCallback(() => {
+    clear();
+  }, [clear]);
+
+  const onTouchEnd = useCallback(
+    (e: TouchEvent) => {
+      clear();
+      if (firedRef.current && suppressClickOnFire) {
+        // Block the synthetic click that follows touchend; the long-press
+        // already handled the gesture.
+        e.preventDefault();
+      }
+    },
+    [clear, suppressClickOnFire],
+  );
+
+  const onTouchCancel = useCallback(
+    (_e: TouchEvent) => {
+      clear();
+    },
+    [clear],
+  );
+
+  return { onTouchStart, onTouchMove, onTouchEnd, onTouchCancel };
+}