fix(ui): pin the in-game ad-banner height on a short viewport

The ad banner lives inside the grown game header (`.nav.grow`, portrait
game only). That rule carried `flex: 1 1 auto`, so on a short viewport the
flex algorithm shrank the header — clipping the banner (`.ad` is
`overflow:hidden`) — *and* the board's scroll area at the same time. The
banner and the board ended up splitting the vertical squeeze.

Drop the shrink (`flex: 1 0 auto`): the header still grows into spare height
(banner under the title, board pinned to the bottom), but on a short viewport
it holds its natural height and the board's own scroll (`.stage`) absorbs the
whole squeeze. The banner now keeps a constant height.

Portrait-only: in landscape and on every other screen `.nav` is already
`flex: 0 0 auto`, so the banner never shrank there.

Verified in the mock UI (portrait, live banner): at 440px tall the banner
held 30px (was clipped to 14px) while the board scrolled; the tall-viewport
layout is unchanged. Full UI suite green locally (check, 272 unit, build,
bundle-size, 140 e2e).

ui/src/components/Header.svelte

@@ -57,7 +57,10 @@
     -webkit-user-select: none;
   }
   .nav.grow {
-    flex: 1 1 auto;
+    /* Grow into spare height (banner under the title, the board pinned to the bottom), but
+       never shrink: on a short viewport the banner keeps its height and the board's own
+       scroll (.stage) absorbs the squeeze, instead of the banner and the board splitting it. */
+    flex: 1 0 auto;
   }
   .bar {
     display: flex;