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>

pyproject.toml

@@ -28,6 +28,8 @@ dependencies = [
 dev = [
     "pytest>=7.4.0",
     "pytest-xdist>=3.5.0",
+    "pytest-playwright>=0.4.0",
+    "requests>=2.31.0",
     "ruff>=0.1.0",
     "filelock>=3.13.0",
 ]
@@ -56,3 +58,6 @@ python_files = "test_*.py"
 python_classes = "Test*"
 python_functions = "test_*"
 addopts = "--durations=20 -n auto"
+markers = [
+    "e2e: end-to-end browser tests (run with -n 0 to disable parallel execution)",
+]