UI: gesture & history polish — pinch/swipe fix, wider back-swipe, nudge align, history overscroll

- Stop a two-finger pinch-out from also opening the history: the board wrapper arms its
  open/close pull only while a single pointer is down (a 2nd finger is a pinch, owned by Board).
- Widen the edge-swipe-back activation band to the left half of the viewport (was 20%).
- Align a chat nudge by sender like a bubble — your own to the right, the opponent's to the
  left (only the alignment changes).
- Kill the iOS rubber-band inside the history drawer (overscroll-behavior: none).

e2e: a two-finger pinch does not open the history; a back-swipe from the left half navigates back.

ui/src/components/Screen.svelte

@@ -36,11 +36,11 @@
   // Edge-swipe back: a rightward drag begun in the left band returns to `back`, the standard
   // mobile gesture (instant on release — the route slide plays the animation). Listened at the
   // window in the CAPTURE phase so the board's own pointer handlers (which capture/stop the
-  // event) can never swallow it; touch/pen only. The band is a fraction of the viewport width
-  // (EDGE_FRACTION — widen/narrow there). A hit-test keeps the wider band clear of the gestures
-  // it would otherwise hijack: the rack (tile lift/reorder), a draggable pending tile, a
-  // zoomed-in board (it pans), and text inputs.
-  const EDGE_FRACTION = 0.2;
+  // event) can never swallow it; touch/pen only. The band is the left half of the viewport
+  // width (EDGE_FRACTION — widen/narrow there). A hit-test keeps the wide band clear of the
+  // gestures it would otherwise hijack: the rack (tile lift/reorder), a draggable pending tile,
+  // a zoomed-in board (it pans), and text inputs.
+  const EDGE_FRACTION = 0.5;
   const SWIPE_SKIP = '[data-rack], .cell.pending, .viewport.zoomed, input, textarea, select';
   $effect(() => {
     function onDown(e: PointerEvent) {