Add TestDriver desktop (Electron) E2E test for Printventory

[testdriverai]

What this adds

A TestDriver computer-use test that drives the real Printventory desktop build (electron . / npm start) — not server mode, per the request in #70.

• tests/electron-app.test.js — end-to-end desktop test that:
  1. Provisions a Linux desktop VM
  2. Clones the repo into the sandbox
  3. Does a lean install — PUPPETEER_SKIP_DOWNLOAD=1 + npm install --ignore-scripts to avoid the heavy electron-builder install-app-deps postinstall (that native rebuild + Puppeteer's Chromium download saturated and wedged the sandbox in my earlier attempts)
  4. Rebuilds only better-sqlite3 for Electron's ABI (it's require()d at the top of main.js, so the app can't boot without it)
  5. Launches electron . --no-sandbox with PRINTVENTORY_TEST_SCAN_PATH set (the app's built-in test hook) so the Scan button skips the native folder-picker and scans the bundled tests/test-fixtures/scan-me/*.stl
  6. Drives core flows: window renders → scan directory → models appear in grid → search/filter ("cube") → open a model → metadata/detail panel visible
• .github/workflows/testdriver-desktop.yml — dedicated CI workflow that authenticates with the TD_API_KEY repo secret and runs this test. It installs only the test-runner deps (--ignore-scripts), because the app itself is provisioned inside the sandbox by the test — so it sidesteps the app's own postinstall on the runner.
• Installs vitest + testdriverai as devDeps and removes the leftover demo sample test.

⚠️ Status: WIP — not yet run green

I authored this without a live sandbox to iterate against: the interactive TestDriver session credential got exhausted mid-run (device code … already been used) and I couldn't provision a fresh VM to validate it interactively. The test is syntactically valid and built from the app's actual UI (verified scan-directory-button, search-filter-input, the PRINTVENTORY_TEST_SCAN_PATH test hook, and the bundled fixtures in the repo), but the natural-language element descriptions and the provisioning/timeout steps will likely need one round of tuning on the first real run.

▶️ How to run it (this is the unblock)

Now that TD_API_KEY is set as a repo Actions secret, run it in CI — that's where the secret is actually injected:

1. Actions tab → TestDriver Desktop (Electron) E2E → Run workflow.
2. It authenticates with TD_API_KEY (no expired device code involved) and runs tests/electron-app.test.js.

If the first run needs element/timeout tweaks, re-invoke me on the issue with a working session and I'll iterate to green.

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Note: this repo also has a Playwright E2E suite (tests/full-app-e2e.spec.js etc.) — that's separate from TestDriver and out of scope for me; this PR only adds the TestDriver computer-use coverage.