refactor(ui): harden the board's recenter dependency and gate it on a real bump

The recenter effect declared its dependency with a bare `void recenter`, which a
production minifier could drop as a side-effect-free statement (the e2e runs only
the unminified dev server, so it would not catch that). Read the nonce into a
used variable and gate the pan on it actually changing (`recentered`), so the
reactive dependency is robust to minification and the board pans only on an
explicit hint recenter — never on an incidental re-run such as a viewport resize.

ui/src/game/Board.svelte

@@ -69,12 +69,16 @@
   // toggle or an explicit recenter request (the hint) moves it. A recenter with no zoom change
   // pans straight to `focus`, since the board width is stable and the width-driven tween below
   // has nothing to ride (the case the owner hit: a hint taken while already zoomed in).
-  // The previous zoom state, to tell a zoom toggle from a recenter request. The board always mounts
-  // zoomed-out, so it starts false and is updated to the live zoom on each effect run.
+  // prevZoom/prevRecenter let the effect tell a zoom toggle from a recenter request. The board
+  // always mounts zoomed-out, so prevZoom starts false; both are updated on each effect run.
   let prevZoom = false;
+  let prevRecenter = 0;
   $effect(() => {
     const on = zoomed;
-    void recenter; // re-run on an explicit recenter request even when the zoom state is unchanged
+    // An explicit "scroll to focus now" request (the hint). Read into a variable that is USED below,
+    // so the reactive dependency survives minification (a bare `void recenter` could be dropped) and
+    // an incidental re-run never pans on its own.
+    const rc = recenter;
     const vp = viewport;
     if (!vp) return;
     const f = untrack(() => focus);
@@ -91,11 +95,13 @@
       finalST = Math.max(0, Math.min((f.row + 0.5) * cell - clientH / 2, fullW - clientH));
     }
     const toggled = on !== prevZoom;
+    const recentered = rc !== prevRecenter;
     prevZoom = on;
+    prevRecenter = rc;
     if (!toggled) {
-      // No zoom change: an explicit recenter (the hint) pans straight to the focused word; a stale
-      // run with nothing focused leaves the scroll alone.
-      if (on && f) {
+      // No zoom change: pan straight to the focused word only on an explicit recenter request (the
+      // hint), never on an incidental re-run such as a viewport resize.
+      if (recentered && on && f) {
         vp.scrollLeft = finalSL;
         vp.scrollTop = finalST;
       }