fix(ads): pulse a lone banner message through the fade cycle

A single campaign message (e.g. the default campaign's one message) faded in once
on load and then sat frozen — the rotator only ran the fade/advance cycle when
more than one message existed, so with one message there were no further fades.
Drop the `total > 1` guards: every message now runs the full hold → fade-out →
gap → fade-in cycle, so a lone message pulses (the same message fades back in)
and a lone long message fades at each scroll-loop boundary. Multi-message
rotation is unchanged. Verified by opacity sampling (single message pulses
1→0→gap→0→1 without navigation); the single-message test now asserts the pulse.

ui/src/lib/banner.test.ts

@@ -122,14 +122,16 @@ describe('createBannerRotator', () => {
     r.stop();
   });
 
-  it('shows a single message once and never fades it out', () => {
+  it('pulses a lone message through the fade cycle (fades out, then back in)', () => {
     vi.useFakeTimers();
     const { host, calls, shown } = recordingHost(0);
     const r = createBannerRotator([{ weight: 1, messages: ['solo'] }], host, cfg);
     r.start();
-    vi.advanceTimersByTime(cfg.fadeInMs + cfg.holdMs * 5);
     expect(shown).toEqual(['solo']);
-    expect(calls).not.toContain('hide');
+    vi.advanceTimersByTime(cfg.fadeInMs + cfg.holdMs); // hold elapses -> fade out
+    expect(calls).toContain('hide');
+    vi.advanceTimersByTime(cfg.fadeOutMs + cfg.gapMs + 1); // -> the same message fades back in
+    expect(shown).toEqual(['solo', 'solo']);
     r.stop();
   });
 

ui/src/lib/banner.ts

@@ -85,8 +85,8 @@ export interface Rotator {
 /**
  * createBannerRotator rotates the scheduled messages: each message fades in, holds
  * `holdMs` (an overflowing one scrolls to its right edge and back while under holdMs),
- * then — when more than one message exists — fades out over `fadeOutMs`, waits `gapMs`,
+ * then fades out over `fadeOutMs`, waits `gapMs`, and fades the next one in. A lone
- * and fades the next one in. A single message stays put (a long one keeps scrolling).
+ * message pulses (the same message fades back in) so the cycle keeps its rhythm.
  * Reduce-motion is handled by the host: pass zeroed fade timings and an overflowPx that
  * returns 0, and messages swap instantly without scrolling.
  */
@@ -125,8 +125,9 @@ export function createBannerRotator(
       scrollCycle(over);
       return;
     }
-    if (sched.total > 1) at(config.holdMs, advance);
+    // Hold, then run the fade cycle. A lone message pulses (fades out, then back in) rather than
-    // A single fitting message stays put.
+    // sitting frozen, so the banner keeps its rhythm even with a single campaign message.
+    at(config.holdMs, advance);
   }
 
   function scrollCycle(over: number) {
@@ -134,7 +135,7 @@ export function createBannerRotator(
     at(config.edgePauseMs, () => {
       host.scrollTo(over, dur);
       at(dur + config.edgePauseMs, () => {
-        if (sched.total > 1 && Date.now() - cycleStart >= config.holdMs) {
+        if (Date.now() - cycleStart >= config.holdMs) {
           advance();
         } else {
           host.resetScroll();