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/active-view/map-toggles.svelte

@@ -14,6 +14,7 @@ bottom-tabs bar.
 <script lang="ts">
 	import { onMount } from "svelte";
 	import { i18n } from "$lib/i18n/index.svelte";
+	import { restoreFocus } from "$lib/a11y/restore-focus";
 	import type { MapToggles, GameStateStore } from "$lib/game-state.svelte";
 	import type { WrapMode } from "../../map/world";
 
@@ -83,7 +84,12 @@ bottom-tabs bar.
 		<span aria-hidden="true">⚙</span>
 	</button>
 	{#if open}
-		<div class="surface" role="menu" data-testid="map-toggles-surface">
+		<div
+			class="surface"
+			role="menu"
+			data-testid="map-toggles-surface"
+			use:restoreFocus
+		>
 			<fieldset>
 				<legend>{i18n.t("game.map.toggles.section.objects")}</legend>
 				<label>