fix(ads): fade the scroll rewind + keep the banner strip height constant

Two polish fixes (owner feedback):

- Scroll loop: a long message that scrolled to its right edge rewound with a hard
  jump (no fade). It now runs the same fade as a message change at each rewind:
  fade out at the edge, reset the scroll while hidden, fade the same message back
  in, then scroll again.
- Strip height: during the fade gap the message layer is removed, which let the
  strip collapse by ~1-2px. An always-present invisible spacer now reserves one
  line of height and the message is overlaid absolutely, so the strip height is
  constant whether or not the message is showing.

Verified live: opacity sampling shows a full fade-out → gap → fade-in at each
scroll rewind (~every 6s), and the .ad height stays a single constant value
(30.31px) across the whole cycle including the gap. Loop-fade unit-tested.

ui/src/lib/banner.ts

@@ -138,8 +138,14 @@ export function createBannerRotator(
         if (Date.now() - cycleStart >= config.holdMs) {
           advance();
         } else {
-          host.resetScroll();
-          scrollCycle(over);
+          // Loop the same message with a fade: fade out at the right edge, rewind to the start
+          // while hidden, fade the same message back in, then scroll again — so the rewind gets
+          // the same fade as a message change, not a hard jump.
+          host.hide(config.fadeOutMs);
+          at(config.fadeOutMs + config.gapMs, () => {
+            host.show(lastShown);
+            at(config.fadeInMs, () => scrollCycle(over));
+          });
         }
       });
     });