build(web): add project-local Playwright tooling

- build: pin Playwright Test and MCP with repository-local npm, browser, and artifact paths plus sandboxed headless Chromium configuration.
- test: add a browser smoke test for the login page while keeping Playwright isolated from Vitest.
- docs: document Debian dependency setup, dual Chromium revisions, E2E commands, and project-scoped Codex MCP usage.

.gitignore

@@ -34,6 +34,10 @@ go.work.sum
 # Mise
 mise.local.toml
 
+# Project-local development caches and browser artifacts
+/.cache/
+/.artifacts/
+
 # MyGO configuration
 # Tracking config.example.yaml in version control
 config.yaml