feat(ads): banner under the header, continuous across navigation, robust fades

Banner UX refinements (owner feedback):
- Position: render the banner inside Header (under the title) instead of in
  Screen, so it sits in the same place on every screen. In the game the grown
  nav's spare height now falls below the banner (banner under title, board
  pinned to the bottom) — it no longer jumps to the game area.
- Continuity: move the rotation into a persistent module engine
  (lib/bannerEngine) — the scheduler + timer live outside the components, so a
  navigation (which remounts the view) continues the cycle instead of restarting
  it. Each AdBanner only attaches as the DOM host and resyncs to the live message.
- Fades: a long, scrolling message now fades at both ends. The fade is a
  {#if} transition:fade layer, independent of the scroll (the inner track's
  transform), so the two no longer interfere.

Verified live (mock + Playwright): same position in lobby and game; the cycle
continues across lobby↔game; opacity sampling shows fade-out + fade-in for the
long message. Engine continuity unit-tested.

ui/src/components/Header.svelte

@@ -3,7 +3,9 @@
   import { insideTelegram } from '../lib/telegram';
   import { connection } from '../lib/connection.svelte';
   import { t } from '../lib/i18n/index.svelte';
+  import { app } from '../lib/app.svelte';
   import Spinner from './Spinner.svelte';
+  import AdBanner from './AdBanner.svelte';
 
   let { title, back, grow = false }: { title: string; back?: string; grow?: boolean } = $props();
 
@@ -29,6 +31,16 @@
     <!-- A right-hand spacer balances the back button so the title stays centred. -->
     <span class="spacer"></span>
   </div>
+  <!-- The ad banner lives inside the nav, directly under the title bar, so it sits in the
+       same place on every screen — and in the game (grown nav) the spare height falls below
+       it (banner under the title, board pinned to the bottom). -->
+  {#if app.profile?.banner && app.profile.banner.campaigns.length}
+    <AdBanner
+      campaigns={app.profile.banner.campaigns}
+      timings={app.profile.banner.timings}
+      reduceMotion={app.reduceMotion}
+    />
+  {/if}
 </header>
 
 <style>