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/components/AdBanner.svelte

@@ -116,6 +116,10 @@
 <!-- svelte-ignore a11y_no_static_element_interactions -->
 <!-- svelte-ignore a11y_click_events_have_key_events -->
 <div class="ad" bind:this={viewport} onclick={onExternalLinkClick}>
+  <!-- An always-present, invisible spacer reserves exactly one line of height, so the strip never
+       collapses while the message layer is absent during the fade gap (the message is overlaid
+       absolutely, so its presence/absence does not change the strip height). -->
+  <span class="reserve" aria-hidden="true">&nbsp;</span>
   {#if visible}
     <div class="fadewrap" in:inFade out:fade={{ duration: eff.fadeOutMs }}>
       <div
@@ -131,6 +135,7 @@
 
 <style>
   .ad {
+    position: relative;
     overflow: hidden;
     white-space: nowrap;
     padding: 6px 0;
@@ -142,9 +147,18 @@
     border-top: 1px solid var(--border);
     border-bottom: 1px solid var(--border);
     user-select: none;
-    /* A stable height so the strip does not collapse during the fade gap (the message layer is
-       removed between fade-out and fade-in). */
-    min-height: calc(0.85rem * 1.2 + 12px);
+  }
+  /* Reserves one line of height inside the padding so .ad keeps a constant height even during the
+     fade gap; invisible and non-interactive. */
+  .reserve {
+    visibility: hidden;
+  }
+  /* The message layer is overlaid on the reserved line, so showing/hiding it never resizes .ad. */
+  .fadewrap {
+    position: absolute;
+    top: 6px;
+    left: 0;
+    right: 0;
   }
   .track {
     display: inline-block;