feat(ui): accessibility pass — WCAG 2.2 AA for login/lobby/shell (F2)

Add the a11y foundation and bring login, lobby, and the in-game shell to
WCAG 2.2 AA:

- Primitives: .sr-only + .skip-link (base.css), trapFocus (modal focus
  trap + restore) and restoreFocus (menu focus restore) actions, the
  --color-focus visible ring.
- In-game shell: skip link + focusable main landmark; WAI-ARIA sidebar
  tabs (roving tabindex, arrow/Home/End, tabpanel wiring); menu Escape +
  focus restore (account / view / turn-navigator / map-toggles /
  bottom-tabs); mail compose as a role=dialog modal with a focus trap.
- login / lobby / lobby-create: skip link + main landmark, field labels,
  role=alert / role=status live regions.
- Map canvas: aria-label naming it a visual overview, with its data
  reachable by keyboard via the sidebar inspector and tables (accessible
  alternative; in-canvas keyboard nav deferred).

Gates (chromium-desktop): tests/e2e/a11y-axe.spec.ts scans every
top-level view for WCAG 2.2 AA violations (zero); a11y-keyboard.spec.ts
covers the skip link, menu Escape+restore, and tab roving. Adds
@axe-core/playwright. Docs: ui/docs/a11y.md (+ index). Marks F1 and F2
done in ui/PLAN-finalize.md.

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

ui/frontend/src/routes/games/[id]/+layout.svelte

@@ -46,6 +46,7 @@ fresh.
 	import { onDestroy, onMount, setContext, untrack } from "svelte";
 	import { goto } from "$app/navigation";
 	import { page } from "$app/state";
+	import { i18n } from "$lib/i18n/index.svelte";
 	import Header from "$lib/header/header.svelte";
 	import HistoryBanner from "$lib/header/history-banner.svelte";
 	import Sidebar from "$lib/sidebar/sidebar.svelte";
@@ -528,6 +529,9 @@ fresh.
 </script>
 
 <div class="game-shell" data-testid="game-shell">
+	<a class="skip-link" href="#active-view-host">
+		{i18n.t("common.skip_to_content")}
+	</a>
 	<Header
 		{gameId}
 		{sidebarOpen}
@@ -535,7 +539,12 @@ fresh.
 	/>
 	<HistoryBanner />
 	<div class="body">
-		<main class="active-view-host" data-testid="active-view-host">
+		<main
+			class="active-view-host"
+			id="active-view-host"
+			tabindex="-1"
+			data-testid="active-view-host"
+		>
 			{#if effectiveTool === "calc"}
 				<Calculator />
 			{:else if effectiveTool === "order"}