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/components/Screen.svelte

@@ -2,6 +2,7 @@
   import type { Snippet } from 'svelte';
   import Header from './Header.svelte';
   import AdBanner from './AdBanner.svelte';
+  import { app } from '../lib/app.svelte';
   import { navigate } from '../lib/router.svelte';
 
   // The app-shell layout (all screens): the nav bar grows; the ad strip, content and
@@ -27,12 +28,6 @@
     column?: boolean;
   } = $props();
 
-  // The promotional banner is feature-gated OFF until it is polished after release. The flag is
-  // a compile-time `false`, so the {#if} branch — and with it the AdBanner import and its
-  // banner.ts logic — is dead-code-eliminated from the production bundle. Flip to
-  // true to bring it back.
-  const SHOW_AD_BANNER = false;
-
   // Edge-swipe back: a rightward drag begun in the left band returns to `back`, the standard
   // mobile gesture (instant on release — the route slide plays the animation). Listened at the
   // window in the CAPTURE phase so the board's own pointer handlers (which capture/stop the
@@ -94,7 +89,13 @@
 
 <div class="screen">
   <Header {title} {back} grow={growNav} />
-  {#if SHOW_AD_BANNER}<AdBanner />{/if}
+  {#if app.profile?.banner && app.profile.banner.campaigns.length}
+    <AdBanner
+      campaigns={app.profile.banner.campaigns}
+      timings={app.profile.banner.timings}
+      reduceMotion={app.reduceMotion}
+    />
+  {/if}
   <main class="content" class:scroll class:fill={!growNav} class:column>{@render children?.()}</main>
   {#if tabbar}
     <nav class="tabbar">{@render tabbar()}</nav>