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/pnpm-lock.yaml

@@ -21,6 +21,9 @@ importers:
         specifier: ^8.18.1
         version: 8.18.1
     devDependencies:
+      '@axe-core/playwright':
+        specifier: 4.11.3
+        version: 4.11.3(playwright-core@1.59.1)
       '@bufbuild/protobuf':
         specifier: ^2.12.0
         version: 2.12.0
@@ -90,6 +93,11 @@ packages:
   '@asamuzakjp/css-color@3.2.0':
     resolution: {integrity: sha512-K1A6z8tS3XsmCMM86xoWdn7Fkdn9m6RSVtocUrJYIwZnFVkng/PvkEoWtOWmP+Scc6saYWHWZYbndEEXxl24jw==}
 
+  '@axe-core/playwright@4.11.3':
+    resolution: {integrity: sha512-h/kfksv4F0cVIDlKpT4700OehdRgpvuVskuQ2nb7/JmtWUXpe9ftHAPtwyXGvVSsa6SJ64A9ER7Zrzc/sIvC4w==}
+    peerDependencies:
+      playwright-core: '>= 1.0.0'
+
   '@babel/code-frame@7.29.0':
     resolution: {integrity: sha512-9NhCeYjq9+3uxgdtp20LSiJXJvN0FeCtNGpJxuMFZ1Kv3cWUNb6DOhJwUvcVCzKGR66cw4njwM6hrJLqgOwbcw==}
     engines: {node: '>=6.9.0'}
@@ -462,6 +470,10 @@ packages:
   asynckit@0.4.0:
     resolution: {integrity: sha512-Oei9OH4tRh0YqU3GxhX79dM/mwVgvbZJaSNaRk+bshkj0S5cfHcgYakreBjrHwatXKbz+IoIdYLxrKim2MjW0Q==}
 
+  axe-core@4.11.4:
+    resolution: {integrity: sha512-KunSNx+TVpkAw/6ULfhnx+HWRecjqZGTOyquAoWHYLRSdK1tB5Ihce1ZW+UY3fj33bYAFWPu7W/GRSmmrCGuxA==}
+    engines: {node: '>=4'}
+
   axobject-query@4.1.0:
     resolution: {integrity: sha512-qIj0G9wZbMGNLjLmg1PT6v2mE9AH2zlnADJD/2tC6E00hgmhUOfEB6greHPAfLRSufHqROIUTkw6E+M3lH0PTQ==}
     engines: {node: '>= 0.4'}
@@ -1152,6 +1164,11 @@ snapshots:
       '@csstools/css-tokenizer': 3.0.4
       lru-cache: 10.4.3
 
+  '@axe-core/playwright@4.11.3(playwright-core@1.59.1)':
+    dependencies:
+      axe-core: 4.11.4
+      playwright-core: 1.59.1
+
   '@babel/code-frame@7.29.0':
     dependencies:
       '@babel/helper-validator-identifier': 7.28.5
@@ -1480,6 +1497,8 @@ snapshots:
 
   asynckit@0.4.0: {}
 
+  axe-core@4.11.4: {}
+
   axobject-query@4.1.0: {}
 
   call-bind-apply-helpers@1.0.2: