Add Playwright e2e test infrastructure

Set up browser-based end-to-end testing using pytest-playwright:
- Add playwright-driver and pytest-playwright to nix flake
- Configure PLAYWRIGHT_BROWSERS_PATH for NixOS compatibility
- Create ServerHarness to manage live server for tests
- Add smoke tests for health endpoint and page loading
- Exclude e2e tests from pre-commit hook (require special setup)

Run e2e tests with: pytest tests/e2e/ -v -n 0

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lefthook.yml

@@ -12,4 +12,4 @@ pre-commit:
       run: ruff format --check src/ tests/
     pytest:
       glob: "**/*.py"
-      run: pytest tests/ -q --tb=short
+      run: pytest tests/ --ignore=tests/e2e -q --tb=short