fix(mobile): hand a swipe-locked drag back to the scroll past the swipe's drift limit

A scroll whose first move past the tap slop leaned sideways (a thumb arc) locked as 'swipe' for the whole contact, and SwipeHandler drops a swipe at 50 px of vertical drift, so the gesture did nothing. Past that limit the drag is now a 'y' scroll. Also brings the mobile-handlers fileoverview to the current 500 ms / 50 px limits and notes in the Mobile Guide that a selection pauses the swipe and a wide terminal pans instead.

src/web/public/mobile-handlers.js

@@ -9,7 +9,7 @@
  *   toolbar/accessory bar repositioning, terminal resize on keyboard open/close,
  *   and input scroll-into-view. Uses 100px threshold for iOS address bar drift.
  * - SwipeHandler — Horizontal swipe detection on the terminal area for session switching.
- *   80px minimum distance, 300ms maximum time, 100px max vertical drift.
+ *   80px minimum distance, 500ms maximum time, 50px max vertical drift.
  *
  * All three have init()/cleanup() lifecycle methods. They are re-initialized after SSE
  * reconnect (in handleInit) to prevent stale closures.

src/web/public/terminal-ui.js

@@ -1200,7 +1200,8 @@ Object.assign(CodemanApp.prototype, {
       // slop, so a diagonal drag cannot pan and scroll at the same time. 'swipe'
       // is a sideways drag on a terminal that cannot pan: it is the session
       // swipe's (SwipeHandler, mobile-handlers.js), so it neither scrolls nor
-      // ends as a tap on the session being left. Published per gesture as
+      // ends as a tap on the session being left, until it drifts past the
+      // swipe's vertical limit and becomes a 'y' scroll. Published per gesture as
       // `this._terminalGestureAxis`, which SwipeHandler reads to stand aside
       // for a scroll or a pan.
       let panAxis = null;
@@ -1335,7 +1336,18 @@ Object.assign(CodemanApp.prototype, {
             // fling, so a jittery tap would both position the cursor AND scroll.
             if (!didScroll) return;
             ev.preventDefault();
-            if (panAxis === 'swipe') return;
+            if (panAxis === 'swipe') {
+              // Past the swipe's own vertical drift limit the session swipe has
+              // given up, so the drag is a scroll after all (a thumb arc whose
+              // first move leaned sideways): hand it to 'y'.
+              const drift = typeof SwipeHandler !== 'undefined' ? SwipeHandler.maxVerticalDrift : 50;
+              if (Math.abs(touchY - touchStartY) <= drift) return;
+              panAxis = 'y';
+              this._terminalGestureAxis = 'y';
+              touchLastX = touchX;
+              touchLastY = touchY;
+              return;
+            }
             if (panAxis === 'x') {
               // Pan the container, and touch nothing the vertical path owns —
               // no pixelAccum, no velocity, so touchend cannot turn a sideways