test(e2e): add axe-core accessibility fixture and smoke spec

Adds @axe-core/playwright with a shared fixture providing an `axe`
helper. New a11y.spec.ts runs WCAG 2.1 AA checks on signin, dashboard,
timeline, allocations, resources, and projects pages. Currently reports
violations as warnings — upgrade to hard failures after fixes.

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

apps/web/package.json

@@ -49,6 +49,7 @@
     "zod": "^3.23.8"
   },
   "devDependencies": {
+    "@axe-core/playwright": "^4.11.1",
     "@capakraken/eslint-config": "workspace:*",
     "@capakraken/tsconfig": "workspace:*",
     "@playwright/test": "^1.49.1",