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.

docs/UI_DESIGN.md

@@ -195,16 +195,24 @@ Login uses `Screen`.
   under-board slot shows the **Scores: N** preview. The screen **title** is the variant's
   display name (Scrabble / Скрэббл / Erudite / Эрудит), not a constant "Scrabble".
 
-## Announcement banner (`components/AdBanner.svelte`, `lib/banner.ts`)
+## Advertising banner (`components/AdBanner.svelte`, `lib/banner.ts`)
 
 A one-line inset strip under the nav bar, drawn on a dedicated `--ad-bg` token —
 a subtle accent, a touch darker than the surroundings in the light theme and a touch lighter
 in the dark theme, mapped to Telegram's `secondary_bg_color` inside the Mini App. Content is
-minimal markdown (text + links, escaped + linkified). A parameterised **rotator** drives messages: a fitting message
-holds `holdMs` (default 60 s) then cross-fades to the next; a message wider than the strip
-pauses (`edgePauseMs`), scrolls to its right edge at `scrollPxPerSec`, pauses, and repeats
-until the cycle exceeds `holdMs`. Today a **mock** provider rotates a long and a short
-message; the source becomes a server-driven channel later (see ARCHITECTURE).
+minimal markdown (text + links, escaped + linkified). It is **server-driven**: the campaigns and
+display timings ride the `profile.get` response (`app.profile.banner`, present only for an eligible
+viewer — see ARCHITECTURE §10), so `Screen` renders the strip only when that block is present, and
+a `notify` `banner` event re-fetches the profile to show or hide it in place.
+
+A **smooth weighted round-robin** (`createScheduler`) picks the next message: campaigns compete by
+their weight (each appears its weight share per cycle, evenly interleaved, not at random), and a
+campaign's own messages advance round-robin. The **rotator** then drives one message: it fades in
+(`fadeInMs`), holds `holdMs` (a message wider than the strip pauses `edgePauseMs`, scrolls to its
+right edge at `scrollPxPerSec`, pauses, and repeats while under `holdMs`), then — when more than one
+message exists — fades out (`fadeOutMs`), waits `gapMs`, and fades the next in. A lone message stays
+put (a long one keeps scrolling). All timings are operator-set (`/_gm/banner-settings`). Under
+**reduce-motion** the fades collapse to an instant swap and a long message does not scroll.
 
 ## Result / status iconography (`lib/result.ts`)