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/i18n/locales/en.ts

@@ -8,6 +8,7 @@ const en = {
 	"common.language": "language",
 	"common.loading": "loading…",
 	"common.dismiss": "dismiss",
+	"common.skip_to_content": "skip to main content",
 	"common.browser_not_supported_title": "browser not supported",
 	"common.browser_not_supported_body":
 		"Galaxy requires Ed25519 in WebCrypto. See supported browsers.",
@@ -117,6 +118,8 @@ const en = {
 	"game.shell.history.current_badge": "current",
 	"game.view.map": "map",
 	"game.map.toggles.open": "open map visibility menu",
+	"game.map.aria_label":
+		"galaxy map ({count} planets) — a visual overview; planet, ship-group and route details are available in the sidebar inspector and the tables view",
 	"game.map.toggles.close": "close map visibility menu",
 	"game.map.toggles.section.objects": "Objects",
 	"game.map.toggles.section.planets": "Planets",