Add Playwright accessibility and mobile layout e2e tests

Set up end-to-end test infrastructure with Docker Compose test
environment, Playwright config, and automated global setup/teardown
that seeds a test database and creates fixtures via the API.

Tests cover 11 pages across both dark/light themes for WCAG 2.0 AA
accessibility (axe-core), and across 3 viewports (mobile, tablet,
desktop) for horizontal overflow and touch target validation.

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

frontend/package.json

@@ -11,7 +11,9 @@
     "format:check": "oxfmt --check src/",
     "preview": "vite preview",
     "test": "vitest run",
-    "test:watch": "vitest"
+    "test:watch": "vitest",
+    "test:e2e": "playwright test",
+    "test:e2e:ui": "playwright test --ui"
   },
   "dependencies": {
     "@dnd-kit/core": "6.3.1",
@@ -24,6 +26,8 @@
     "sonner": "2.0.7"
   },
   "devDependencies": {
+    "@axe-core/playwright": "4.11.1",
+    "@playwright/test": "1.58.2",
     "@tailwindcss/vite": "4.1.18",
     "@types/node": "24.10.10",
     "@types/react": "19.2.11",