feat(eslint): add jsx-a11y accessibility rules as warnings

Install eslint-plugin-jsx-a11y and add 24 recommended rules to the
nextjs ESLint config, all set to warn. Baseline: 292 warnings
(207 label-has-associated-control, 52 no-static-element-interactions,
22 click-events-have-key-events, 10 no-autofocus, 1 html-has-lang).

Will be upgraded to errors after Phase 5c fixes core components.

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

tooling/eslint/package.json

@@ -18,5 +18,8 @@
   "peerDependencies": {
     "eslint": "^9.0.0",
     "typescript": "^5.0.0"
+  },
+  "devDependencies": {
+    "eslint-plugin-jsx-a11y": "^6.10.2"
   }
 }