feat(ads): client banner rotation, fade UX & live toggle (PR2)

Consume the server-driven banner block (PR1) in the UI and retire the gate.

- banner.ts: createScheduler — a smooth weighted round-robin over campaigns (each
  appears its weight share per cycle, evenly interleaved) with round-robin over a
  campaign's messages; the rotator drives fade-in -> hold/scroll -> fade-out -> gap
  -> fade-in, a lone message stays put, reduce-motion swaps instantly without scroll.
- model.ts/codec.ts: Profile.banner (Banner/BannerCampaign/BannerTimings) decoded
  from the fbs block.
- Screen.svelte: drop the compile-time SHOW_AD_BANNER; render AdBanner from
  app.profile.banner (campaigns + timings + reduceMotion).
- AdBanner.svelte: opacity-driven fades + scroll host; the rotator is recreated when
  the campaigns/timings change (a `banner` notify re-fetch swaps them in place).
- app.svelte.ts: on the `notify` `banner` sub-kind, refreshProfile() so the banner
  shows/hides in place.
- tests: scheduler distribution + round-robin, the fade sequence, single-message,
  reduce-motion, stop(); codec banner decode. UI_DESIGN.md + trackers updated.

ui/src/lib/codec.test.ts

@@ -12,6 +12,7 @@ import {
   decodeLinkResult,
   decodeMatchResult,
   decodeOutgoingList,
+  decodeProfile,
   decodeSession,
   decodeFeedbackState,
   decodeFeedbackUnread,
@@ -449,6 +450,40 @@ describe('codec', () => {
     expect(r.game?.id).toBe('g-open');
     expect(r.game?.status).toBe('open');
   });
+
+  it('decodes the profile banner block (campaigns + timings)', () => {
+    const b = new Builder(256);
+    const uid = b.createString('u-1');
+    const m0 = b.createString('promo-en');
+    const msgs = fb.BannerCampaign.createMessagesVector(b, [m0]);
+    const camp = fb.BannerCampaign.createBannerCampaign(b, 3, msgs);
+    const camps = fb.BannerInfo.createCampaignsVector(b, [camp]);
+    const banner = fb.BannerInfo.createBannerInfo(b, camps, 60000, 5000, 40, 1000, 250, 1000);
+    fb.Profile.startProfile(b);
+    fb.Profile.addUserId(b, uid);
+    fb.Profile.addBanner(b, banner);
+    b.finish(fb.Profile.endProfile(b));
+
+    const p = decodeProfile(b.asUint8Array());
+    expect(p.banner?.campaigns).toEqual([{ weight: 3, messages: ['promo-en'] }]);
+    expect(p.banner?.timings).toEqual({
+      holdMs: 60000,
+      edgePauseMs: 5000,
+      scrollPxPerSec: 40,
+      fadeOutMs: 1000,
+      gapMs: 250,
+      fadeInMs: 1000,
+    });
+  });
+
+  it('leaves the profile banner undefined when absent', () => {
+    const b = new Builder(64);
+    const uid = b.createString('u-1');
+    fb.Profile.startProfile(b);
+    fb.Profile.addUserId(b, uid);
+    b.finish(fb.Profile.endProfile(b));
+    expect(decodeProfile(b.asUint8Array()).banner).toBeUndefined();
+  });
 });
 
 // The live play loop exchanges alphabet indices, mapped through the per-variant