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();
   });