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"}

ui/frontend/src/routes/lobby/+page.svelte

@@ -265,7 +265,8 @@
 	});
 </script>
 
-<main>
+<a class="skip-link" href="#main-content">{i18n.t("common.skip_to_content")}</a>
+<main id="main-content" tabindex="-1">
 	<header>
 		<h1>{i18n.t("lobby.title")}</h1>
 		<p>
@@ -297,7 +298,7 @@
 	<section data-testid="lobby-my-games-section">
 		<h2>{i18n.t("lobby.section.my_games")}</h2>
 		{#if listsLoading}
-			<p>{i18n.t("lobby.list_loading")}</p>
+			<p role="status">{i18n.t("lobby.list_loading")}</p>
 		{:else if myGames.length === 0}
 			<p data-testid="lobby-my-games-empty">{i18n.t("lobby.my_games.empty")}</p>
 		{:else}
@@ -323,7 +324,7 @@
 	<section data-testid="lobby-invitations-section">
 		<h2>{i18n.t("lobby.section.invitations")}</h2>
 		{#if listsLoading}
-			<p>{i18n.t("lobby.list_loading")}</p>
+			<p role="status">{i18n.t("lobby.list_loading")}</p>
 		{:else if invitations.length === 0}
 			<p data-testid="lobby-invitations-empty">{i18n.t("lobby.invitations.empty")}</p>
 		{:else}
@@ -357,7 +358,7 @@
 	<section data-testid="lobby-applications-section">
 		<h2>{i18n.t("lobby.section.applications")}</h2>
 		{#if listsLoading}
-			<p>{i18n.t("lobby.list_loading")}</p>
+			<p role="status">{i18n.t("lobby.list_loading")}</p>
 		{:else if applications.length === 0}
 			<p data-testid="lobby-applications-empty">{i18n.t("lobby.applications.empty")}</p>
 		{:else}
@@ -416,7 +417,7 @@
 	<section data-testid="lobby-public-games-section">
 		<h2>{i18n.t("lobby.section.public_games")}</h2>
 		{#if listsLoading}
-			<p>{i18n.t("lobby.list_loading")}</p>
+			<p role="status">{i18n.t("lobby.list_loading")}</p>
 		{:else if publicGames.length === 0}
 			<p data-testid="lobby-public-games-empty">{i18n.t("lobby.public_games.empty")}</p>
 		{:else}

ui/frontend/src/routes/lobby/create/+page.svelte

@@ -125,7 +125,8 @@
 	});
 </script>
 
-<main>
+<a class="skip-link" href="#main-content">{i18n.t("common.skip_to_content")}</a>
+<main id="main-content" tabindex="-1">
 	<h1>{i18n.t("lobby.create.title")}</h1>
 	{#if configError !== null}
 		<p role="alert" data-testid="lobby-create-config-error">{configError}</p>

ui/frontend/src/routes/login/+page.svelte

@@ -133,7 +133,8 @@
 	}
 </script>
 
-<main>
+<a class="skip-link" href="#main-content">{i18n.t("common.skip_to_content")}</a>
+<main id="main-content" tabindex="-1">
 	<header>
 		<h1>{i18n.t("login.title")}</h1>
 		<div class="language-picker">
@@ -166,7 +167,7 @@
 					stroke-width="1.5"
 				/>
 			</svg>
-			<label class="visually-hidden" for="login-language-select">
+			<label class="sr-only" for="login-language-select">
 				{i18n.t("common.language")}
 			</label>
 			<select
@@ -303,18 +304,6 @@
 		padding: 0.25rem 0.5rem;
 	}
 
-	.visually-hidden {
-		position: absolute;
-		width: 1px;
-		height: 1px;
-		padding: 0;
-		margin: -1px;
-		overflow: hidden;
-		clip: rect(0, 0, 0, 0);
-		white-space: nowrap;
-		border: 0;
-	}
-
 	form {
 		display: flex;
 		flex-direction: column;