feat(ads): carry the banner scroll position across navigation

Per the owner's idea: instead of moving the banner out of the per-screen header
(which would change its position), remember the banner's "life stage" and resume
it on the next screen. The engine already keeps the message + rotation timing;
this adds the scroll offset:

- bannerEngine tracks the in-flight scroll (target, duration, start). On attach,
  if a scroll is still running, it computes the current offset and calls the new
  host's resumeScroll(fromTx, toPx, remaining) — the view jumps to the carried
  offset and continues to the end over the remaining time, instead of restarting
  at the left.
- A finished scroll is left at its end; the rotator's own loop then takes over.

Verified: spot-checked in the browser (a long message at offset -785 resumes at
-788 on the next screen, not 0) and unit-tested (attach mid-scroll calls
resumeScroll with a partial offset and the remaining duration).

ui/src/components/AdBanner.svelte

@@ -70,6 +70,16 @@
       txDur = durationMs;
       tx = -toPx;
     },
+    resumeScroll(fromTx, toPx, durationMs) {
+      // Jump to the carried-over offset instantly, then continue to the end over the remaining
+      // time — so a long message keeps its scroll position across a navigation.
+      txDur = 0;
+      tx = fromTx;
+      requestAnimationFrame(() => {
+        txDur = durationMs;
+        tx = -toPx;
+      });
+    },
   };
 
   // Attach to the persistent engine on mount, detach on unmount (without stopping it). Declared