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.

PLAN.md

@@ -1428,9 +1428,9 @@ cannot submit; three-way admin filter.
       docker-network caddy hop `172.18.0.x` for chat moderation, and bucketing the gateway's
       per-IP rate limiter on it). Correct + spoof-safe in **both** contours (prod has no host
       caddy → public clients untrusted → real peer used). `peerIP` unit-tested.
-    - **Ad banner** gated **off** behind a compile-time `SHOW_AD_BANNER=false` in `Screen.svelte`
-      — the `{#if}` branch, the `AdBanner` import and `banner.ts` are tree-shaken out of the prod
-      bundle (code kept for post-release polish).
+    - **Ad banner** was gated off here behind `SHOW_AD_BANNER=false`; the **AD** phase
+      (PRERELEASE.md) later turned it into the real server-driven advertising network and removed
+      the gate (`Screen` renders it from `app.profile.banner`).
     - **Landing** Telegram entry is now just the **64px logo** (clickable, no button/caption).
     - **TG-fullscreen header** reworked again: title + menu are one **centred pair** (hamburger
       right of the title) pinned to the **bottom** of the TG nav band, lining up with Telegram's