UI: widen the edge-swipe-back activation band

Replace the very-narrow 24px edge-swipe trigger with a left band of ~20% of the
viewport width (EDGE_FRACTION) so the back-swipe is easy to reach, keeping the
simple instant-navigate behaviour (the route slide plays the animation). A
hit-test keeps the wider band clear of the rack, a draggable pending tile, a
zoomed-in board and text inputs, so it never hijacks those drags.

Test: e2e (Chromium+WebKit) — the band swipe returns to the lobby; a swipe
starting on the rack does not navigate.

ui/src/components/Screen.svelte

@@ -33,13 +33,19 @@
   // true to bring it back.
   const SHOW_AD_BANNER = false;
 
-  // Edge-swipe back: a left-edge rightward drag returns to `back`, the standard
-  // mobile gesture. Listened at the window in the CAPTURE phase so the board's own pointer
-  // handlers (which capture/stop the event) can never swallow it; armed only from the very
-  // left edge (<=24px), touch/pen only, so it never competes with the board's gestures.
+  // 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;
+  const SWIPE_SKIP = '[data-rack], .cell.pending, .viewport.zoomed, input, textarea, select';
   $effect(() => {
     function onDown(e: PointerEvent) {
-      if (!back || e.pointerType === 'mouse' || e.clientX > 24) return;
+      if (!back || e.pointerType === 'mouse' || e.clientX > window.innerWidth * EDGE_FRACTION) return;
+      if (document.elementFromPoint(e.clientX, e.clientY)?.closest(SWIPE_SKIP)) return;
       const x0 = e.clientX;
       const y0 = e.clientY;
       const onUp = (ev: PointerEvent) => {