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/lib/header/account-menu.svelte

@@ -15,6 +15,7 @@ Sessions and Theme) take over.
 	} from "$lib/i18n/index.svelte";
 	import { session } from "$lib/session-store.svelte";
 	import { theme, type ThemeChoice } from "$lib/theme/theme.svelte";
+	import { restoreFocus } from "$lib/a11y/restore-focus";
 
 	const THEME_CHOICES: ReadonlyArray<{ id: ThemeChoice; key: TranslationKey }> = [
 		{ id: "system", key: "game.shell.menu.theme_system" },
@@ -79,7 +80,12 @@ Sessions and Theme) take over.
 		⚙
 	</button>
 	{#if open}
-		<div class="surface" role="menu" data-testid="account-menu-list">
+		<div
+			class="surface"
+			role="menu"
+			data-testid="account-menu-list"
+			use:restoreFocus
+		>
 			<button type="button" role="menuitem" data-testid="account-menu-settings" disabled>
 				{i18n.t("game.shell.menu.settings")}
 			</button>

ui/frontend/src/lib/header/turn-navigator.svelte

@@ -17,6 +17,7 @@ Selecting a row calls `gameState.viewTurn(N)`; the row that matches
 <script lang="ts">
 	import { getContext, onMount } from "svelte";
 	import { i18n } from "$lib/i18n/index.svelte";
+	import { restoreFocus } from "$lib/a11y/restore-focus";
 	import {
 		GAME_STATE_CONTEXT_KEY,
 		type GameStateStore,
@@ -139,7 +140,12 @@ Selecting a row calls `gameState.viewTurn(N)`; the row that matches
 		→
 	</button>
 	{#if open}
-		<div class="surface" role="menu" data-testid="turn-navigator-list">
+		<div
+			class="surface"
+			role="menu"
+			data-testid="turn-navigator-list"
+			use:restoreFocus
+		>
 			{#each turns as turn (turn)}
 				<button
 					type="button"

ui/frontend/src/lib/header/view-menu.svelte

@@ -16,6 +16,7 @@ polishes microcopy.
 	import { goto } from "$app/navigation";
 	import { i18n, type TranslationKey } from "$lib/i18n/index.svelte";
 	import { mailStore } from "$lib/mail-store.svelte";
+	import { restoreFocus } from "$lib/a11y/restore-focus";
 
 	type Props = { gameId: string };
 	let { gameId }: Props = $props();
@@ -81,7 +82,12 @@ polishes microcopy.
 		<span class="icon-hamburger" aria-hidden="true">☰</span>
 	</button>
 	{#if open}
-		<div class="surface" role="menu" data-testid="view-menu-list">
+		<div
+			class="surface"
+			role="menu"
+			data-testid="view-menu-list"
+			use:restoreFocus
+		>
 			<button
 				type="button"
 				role="menuitem"