fix(mobile): page transitions hung four seconds on every navigation to a lazy page

The transition waited for the new page to replace its route fallback and
polled for that with requestAnimationFrame. While a view transition waits
for its update the browser renders no frames, so the callback never fired
and every navigation to a page whose chunk was still loading sat frozen
until the browser gave up after four seconds. The wait now polls with a
timer: from Vacay to the journal takes 0.45 s instead of 4.1 s with an
empty cache.

client/src/mobile/routeTransition.ts

@@ -44,13 +44,19 @@ function canAnimate(): boolean {
   );
 }
 
-/** Resolves once no route fallback is on screen, or after SETTLE_MS. */
+/**
+ * Resolves once no route fallback is on screen, or after SETTLE_MS. Polled with
+ * a timer, never requestAnimationFrame: while a view transition waits for its
+ * update, the browser renders no frames, so a frame callback never came and the
+ * transition hung until the browser gave up after four seconds, on every
+ * navigation.
+ */
 function settled(): Promise<void> {
   const began = performance.now();
   return new Promise((resolve) => {
     const check = () => {
       if (!document.querySelector('[data-route-fallback]') || performance.now() - began > SETTLE_MS) resolve();
-      else requestAnimationFrame(check);
+      else window.setTimeout(check, 16);
     };
     check();
   });