test(#118): add axe-core wcag2a/wcag2aa accessibility checks to E2E suite

Installs @axe-core/playwright and adds e2e/accessibility.spec.ts covering:
- home, persons, admin (authenticated via stored admin session)
- login (unauthenticated context)

Uses wcag2a + wcag2aa tags. Violations are logged with impact level and
node count before the assertion fails, so the first run against the live
stack will produce a clear inventory of any issues to fix or exclude.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -26,6 +26,7 @@
 		"pdfjs-dist": "^5.5.207"
 	},
 	"devDependencies": {
+		"@axe-core/playwright": "^4.11.1",
 		"@eslint/compat": "^1.4.0",
 		"@eslint/js": "^9.39.1",
 		"@inlang/paraglide-js": "^2.5.0",