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/.gitignore

@@ -12,6 +12,12 @@ dist
 dist-ssr
 *.local
 
+# Playwright
+e2e/.fixtures.json
+e2e/screenshots/
+test-results/
+playwright-report/
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json