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/tests/e2e/a11y-keyboard.spec.ts

@@ -0,0 +1,66 @@
+// F2 — keyboard-only navigation coverage for the in-game shell.
+// Runs on chromium-desktop (a stable ≥1024px viewport where the sidebar
+// is visible); the behaviours under test are engine-independent.
+
+import { expect, test, type Page } from "@playwright/test";
+
+const SESSION_ID = "f2-a11y-keyboard-session";
+const GAME_ID = "10101010-1010-1010-1010-101010101010";
+
+async function bootShell(page: Page): Promise<void> {
+	await page.goto("/__debug/store");
+	await expect(page.getByTestId("debug-store-ready")).toBeVisible();
+	await page.waitForFunction(() => window.__galaxyDebug?.ready === true);
+	await page.evaluate(() => window.__galaxyDebug!.clearSession());
+	await page.evaluate(
+		(id) => window.__galaxyDebug!.setDeviceSessionId(id),
+		SESSION_ID,
+	);
+	await page.goto(`/games/${GAME_ID}/map`);
+	await expect(page.getByTestId("game-shell")).toBeVisible();
+	await expect(page.getByTestId("active-view-map")).toBeVisible();
+}
+
+test.describe("keyboard navigation", () => {
+	test.beforeEach(({}, testInfo) => {
+		test.skip(
+			testInfo.project.name !== "chromium-desktop",
+			"keyboard specs run on chromium-desktop",
+		);
+	});
+
+	test("skip link is the first focusable and jumps to main content", async ({
+		page,
+	}) => {
+		await bootShell(page);
+		await page.keyboard.press("Tab");
+		await expect(page.locator(".skip-link")).toBeFocused();
+		await page.keyboard.press("Enter");
+		await expect(page.locator("#active-view-host")).toBeFocused();
+	});
+
+	test("Escape closes the account menu and returns focus to its trigger", async ({
+		page,
+	}) => {
+		await bootShell(page);
+		await page.getByTestId("account-menu-trigger").click();
+		await expect(page.getByTestId("account-menu-list")).toBeVisible();
+		// Move focus into the menu, then dismiss with Escape.
+		await page.getByTestId("account-menu-theme-select").focus();
+		await page.keyboard.press("Escape");
+		await expect(page.getByTestId("account-menu-list")).toBeHidden();
+		await expect(page.getByTestId("account-menu-trigger")).toBeFocused();
+	});
+
+	test("sidebar tabs move with the arrow keys (roving)", async ({ page }) => {
+		await bootShell(page);
+		const calculator = page.getByTestId("sidebar-tab-calculator");
+		const inspector = page.getByTestId("sidebar-tab-inspector");
+		await calculator.click();
+		await expect(calculator).toHaveAttribute("aria-selected", "true");
+		await calculator.focus();
+		await page.keyboard.press("ArrowRight");
+		await expect(inspector).toBeFocused();
+		await expect(inspector).toHaveAttribute("aria-selected", "true");
+	});
+});