Run the desktop app on native PHP

[priethor]

What

Part of #196

Desktop now boots the bundled WordPress site with host PHP and SQLite, not WordPress Playground. The snapshot step builds a real WordPress install, seeds Cortext, and includes the small runtime pieces the app needs for routing and desktop auth. This PR also adds a Playwright Electron smoke test and a GitHub Actions workflow for desktop-related changes.

Why

Playground was the bottleneck with the seeded DataViews workspace open. Running both paths on the same machine, against the same data, made the difference pretty clear:

Metric	Playground (PHP-WASM)	Native PHP
REST request	~600-1000 ms	~30-60 ms
Shell paint	~1 s	~80 ms
Cold launch (extract + boot)	~5-10 s	~3-5 s
Warm launch to ready	~1-2 s	< 1 s
Snapshot build	~30 s	~18 s
Snapshot size	~40 MB	~30 MB

How

• Starting php -S from Electron and waiting for it before loading the Cortext admin page.
• Building snapshots from cached downloads of WordPress, wp-cli, and sqlite-database-integration.
• Generating a fresh wp-config.php with random salts, CORTEXT_DESKTOP, WP_HOME, and WP_SITEURL.
• Copying the desktop router and desktop-only autologin mu-plugin into the snapshot.
• Removing blueprint.json and the Playground client from both build and runtime.
• Running the desktop smoke test only when desktop-related paths change.

Trade-offs

• The app now needs php on the host's PATH. Bundled PHP per architecture is still future work, not part of this PR.
• The autologin mu-plugin fully bypasses auth_redirect. It is only for desktop snapshots and must never ship to a public WordPress site.
• The smoke test stays narrow for now: extract, boot, mount the Cortext shell. It does not cover editor flows yet.

Testing Instructions

1. Make sure PHP 8.1+ with pdo_sqlite is available on PATH. On macOS, brew install php is enough.
2. From the repo root, run npm install && npm --prefix apps/desktop install && npm --prefix apps/desktop run snapshot. The build takes about 18 seconds; later runs use cached downloads.
3. Launch with npm --prefix apps/desktop start. The window should reach Cortext admin in well under a second.
4. Confirm no wp-login screen appears.
5. Run npm --prefix apps/desktop run test:e2e. It should finish in about 7 seconds once the snapshot exists.