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/app.svelte.ts

@@ -276,6 +276,11 @@ function openStream(): void {
         if (e.sub === 'admin_reply') {
           app.feedbackReplyUnread = true;
         }
+        // The viewer's ad-banner eligibility may have changed (paid/hints/no_banner): re-fetch
+        // the profile, which carries the banner block, so the banner shows or hides in place.
+        if (e.sub === 'banner') {
+          void refreshProfile();
+        }
         void refreshNotifications();
       }
     },
@@ -300,6 +305,20 @@ function scheduleReconnect(): void {
   }, 4000);
 }
 
+/**
+ * refreshProfile re-fetches the account profile (which carries the ad-banner block),
+ * after a `banner` notify signals the viewer's banner eligibility changed. Best-effort:
+ * a transient failure leaves the previous profile in place.
+ */
+export async function refreshProfile(): Promise<void> {
+  if (!app.session) return;
+  try {
+    app.profile = await gateway.profileGet();
+  } catch {
+    // Best-effort; the banner just stays as it was until the next fetch.
+  }
+}
+
 /**
  * refreshNotifications recomputes the badge count (incoming friend requests).
  * Authoritative poll, complementing the live 'notify' push. Game invitations have