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.

.codex/config.toml

@@ -0,0 +1,22 @@
+[mcp_servers.playwright]
+command = "mise"
+args = [
+  "exec",
+  "--",
+  "node",
+  "node_modules/@playwright/mcp/cli.js",
+  "--headless",
+  "--browser",
+  "chromium",
+  "--sandbox",
+  "--isolated",
+  "--output-dir",
+  "../.artifacts/playwright-mcp",
+  "--output-max-size",
+  "104857600",
+]
+cwd = "web"
+enabled = true
+required = false
+startup_timeout_sec = 30
+tool_timeout_sec = 120