fix(ui): pin the mobile game shell to the viewport

The app-shell migration surfaced a mobile-only e2e failure: taps on the
bottom-tab bar, the map-toggles menu, and the planet sheet were
intercepted by sibling elements despite the targets being on top.

Root cause: `.game-shell` used `min-height: 100vh`, so sub-pixel content
overflowed the viewport and made the document scrollable. On mobile that
scroll toggles the browser's dynamic toolbar, which resizes the viewport
and every `position: fixed` overlay (their sizes derive from `100vh`)
mid-gesture — defeating Playwright's actionability hit-test, and making
the real controls jittery to tap.

Pin the shell with `position: fixed; inset: 0` on the mobile breakpoint
so it leaves document flow: the document can no longer scroll, the
toolbar stays put, the viewport and overlays stay stable, and the
active-view area remains the single internal scroll region. Desktop is
unchanged (the rule is scoped to max-width: 767.98px).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/src/lib/game/game-shell.svelte

@@ -671,6 +671,22 @@ return to the lobby still disposes the stores via `onDestroy`.
 		overflow-y: auto;
 	}
 	@media (max-width: 767.98px) {
+		/*
+		Pin the shell to the viewport on mobile and take it out of
+		document flow. With `min-height: 100vh` the shell can overflow
+		the viewport by a few sub-pixels, which makes the document
+		scrollable; scrolling then toggles the mobile browser's dynamic
+		toolbar, resizing the viewport and the `position: fixed` overlays
+		(map-toggles menu, bottom-tab drawer, planet sheet) mid-gesture.
+		`position: fixed; inset: 0` keeps the viewport — and those
+		overlays — stable, leaving the active-view area as the single
+		internal scroll region.
+		*/
+		.game-shell {
+			position: fixed;
+			inset: 0;
+			min-height: 0;
+		}
 		.body {
 			padding-bottom: 3.25rem;
 		}