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/docs/README.md

@@ -12,6 +12,9 @@ lives in [`../PLAN.md`](../PLAN.md), the active web finalization in
 - [design-system.md](design-system.md) — the design tokens (colour /
   spacing / typography), the light/dark theming mechanism, and the
   component migration conventions.
+- [a11y.md](a11y.md) — the WCAG 2.2 AA approach: axe + keyboard test
+  gates, the shared a11y primitives, coverage by area, and the map-canvas
+  alternative.
 - [navigation.md](navigation.md) — routes, the sidebar tabs, and the
   state-preservation rules across view/tab switches.
 - [storage.md](storage.md) — the `KeyStore` and `Cache` abstractions and