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/model.ts

@@ -138,6 +138,32 @@ export interface Profile {
   isGuest: boolean;
   /** Confine notifications to the in-app stream (no out-of-app platform push). */
   notificationsInAppOnly: boolean;
+  /** The advertising-banner block, present only for a viewer eligible to see it. */
+  banner?: Banner;
+}
+
+/** Banner is the advertising-banner block of an eligible viewer's profile. */
+export interface Banner {
+  campaigns: BannerCampaign[];
+  timings: BannerTimings;
+}
+
+/** BannerCampaign is one campaign in the rotation feed. */
+export interface BannerCampaign {
+  /** GCD-reduced show weight for the client's smooth weighted round-robin. */
+  weight: number;
+  /** Messages in display order, already resolved to the viewer's bot language. */
+  messages: string[];
+}
+
+/** BannerTimings are the global banner display timings (ms, except scrollPxPerSec). */
+export interface BannerTimings {
+  holdMs: number;
+  edgePauseMs: number;
+  scrollPxPerSec: number;
+  fadeOutMs: number;
+  gapMs: number;
+  fadeInMs: number;
 }
 
 /** BlockStatus is the caller's current manual block, fetched after any call reports