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/theme/base.css

@@ -27,3 +27,41 @@ body {
 ::selection {
 	background: var(--color-accent-subtle);
 }
+
+/*
+ * Visually-hidden content that stays available to assistive tech. Use
+ * for labels/announcements a sighted user gets from layout or icons.
+ */
+.sr-only {
+	position: absolute;
+	width: 1px;
+	height: 1px;
+	padding: 0;
+	margin: -1px;
+	overflow: hidden;
+	clip: rect(0, 0, 0, 0);
+	white-space: nowrap;
+	border: 0;
+}
+
+/*
+ * Skip link: visually hidden until focused, then slides into the
+ * top-left so a keyboard user can jump straight to the main content.
+ * Each layout renders one as its first focusable element, targeting a
+ * `tabindex="-1"` main region.
+ */
+.skip-link {
+	position: absolute;
+	left: var(--space-2);
+	top: -4rem;
+	z-index: 100;
+	padding: var(--space-2) var(--space-3);
+	background: var(--color-surface-overlay);
+	color: var(--color-text);
+	border: 1px solid var(--color-border);
+	border-radius: var(--radius-sm);
+	transition: top 120ms ease;
+}
+.skip-link:focus-visible {
+	top: var(--space-2);
+}