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.ts

@@ -9,6 +9,8 @@ import { indexForLetter, letterForIndex, setAlphabet, type AlphabetEntryWire } f
 import type { PlacedTile } from './client';
 import type {
   AccountRef,
+  Banner,
+  BannerCampaign,
   BlockStatus,
   ChatMessage,
   EvalResult,
@@ -316,6 +318,33 @@ export function decodeProfile(buf: Uint8Array): Profile {
     blockFriendRequests: p.blockFriendRequests(),
     isGuest: p.isGuest(),
     notificationsInAppOnly: p.notificationsInAppOnly(),
+    banner: decodeBanner(p),
+  };
+}
+
+// decodeBanner projects the optional advertising-banner block of a Profile, or
+// undefined when the viewer is not eligible (the field is absent).
+function decodeBanner(p: fb.Profile): Banner | undefined {
+  const b = p.banner();
+  if (!b) return undefined;
+  const campaigns: BannerCampaign[] = [];
+  for (let i = 0; i < b.campaignsLength(); i++) {
+    const c = b.campaigns(i);
+    if (!c) continue;
+    const messages: string[] = [];
+    for (let j = 0; j < c.messagesLength(); j++) messages.push(c.messages(j));
+    campaigns.push({ weight: c.weight(), messages });
+  }
+  return {
+    campaigns,
+    timings: {
+      holdMs: b.holdMs(),
+      edgePauseMs: b.edgePauseMs(),
+      scrollPxPerSec: b.scrollPxPerSec(),
+      fadeOutMs: b.fadeOutMs(),
+      gapMs: b.gapMs(),
+      fadeInMs: b.fadeInMs(),
+    },
   };
 }