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/mail/compose.svelte

@@ -10,6 +10,7 @@ surfaces the resulting 403 inline.
 	import { getContext } from "svelte";
 
 	import { i18n } from "$lib/i18n/index.svelte";
+	import { trapFocus } from "$lib/a11y/focus-trap";
 	import { mailStore } from "$lib/mail-store.svelte";
 	import {
 		RENDERED_REPORT_CONTEXT_KEY,
@@ -51,6 +52,10 @@ surfaces the resulting 403 inline.
 		}
 	});
 
+	function onWindowKeydown(event: KeyboardEvent): void {
+		if (event.key === "Escape") onClose();
+	}
+
 	async function submit(event: SubmitEvent): Promise<void> {
 		event.preventDefault();
 		error = null;
@@ -84,16 +89,30 @@ surfaces the resulting 403 inline.
 	}
 </script>
 
-<div class="overlay" data-testid="mail-compose">
+<svelte:window onkeydown={onWindowKeydown} />
+
+<div
+	class="overlay"
+	data-testid="mail-compose"
+	role="dialog"
+	aria-modal="true"
+	aria-labelledby="mail-compose-title"
+	use:trapFocus
+>
 	<form class="dialog" onsubmit={submit}>
 		<header>
-			<h3>{i18n.t("game.mail.compose_action")}</h3>
-			<button type="button" class="close" onclick={onClose}>×</button>
+			<h3 id="mail-compose-title">{i18n.t("game.mail.compose_action")}</h3>
+			<button
+				type="button"
+				class="close"
+				aria-label={i18n.t("common.dismiss")}
+				onclick={onClose}>×</button
+			>
 		</header>
 
 		<label>
 			{i18n.t("game.mail.compose.target_label")}
-			<select bind:value={kind} data-testid="mail-compose-kind">
+			<select bind:value={kind} data-testid="mail-compose-kind" data-autofocus>
 				<option value="personal">{i18n.t("game.mail.compose.target_personal")}</option>
 				<option value="broadcast">{i18n.t("game.mail.compose.target_broadcast")}</option>
 			</select>
@@ -111,7 +130,7 @@ surfaces the resulting 403 inline.
 		{/if}
 
 		<label>
-			<span class="visually-hidden">{i18n.t("game.mail.subject_placeholder")}</span>
+			<span class="sr-only">{i18n.t("game.mail.subject_placeholder")}</span>
 			<input
 				type="text"
 				bind:value={subject}
@@ -121,7 +140,7 @@ surfaces the resulting 403 inline.
 		</label>
 
 		<label>
-			<span class="visually-hidden">{i18n.t("game.mail.body_placeholder")}</span>
+			<span class="sr-only">{i18n.t("game.mail.body_placeholder")}</span>
 			<textarea
 				bind:value={body}
 				placeholder={i18n.t("game.mail.body_placeholder")}
@@ -229,12 +248,4 @@ surfaces the resulting 403 inline.
 		font-size: 0.85rem;
 		margin: 0;
 	}
-	.visually-hidden {
-		position: absolute;
-		clip: rect(0 0 0 0);
-		clip-path: inset(50%);
-		height: 1px;
-		width: 1px;
-		overflow: hidden;
-	}
 </style>