Fix API test runner to use old headless Chromium mode for WebGL support

Playwright 1.44+ switched to a new headless shell binary that drops WebGL
support. Tests that use Babylon.js (e.g. babylon, objects, materials) were
failing locally on macOS with "WebGL not supported" because flock.scene was
never initialised.

Switching to headless:false with --headless=old restores the original headless
Chromium mode which retains WebGL/WebGL2 support. This is consistent with what
CI has been getting implicitly via the older Playwright build (v1193) it uses.

Also fix tests/tests.html to load mocha and chai from local node_modules
instead of unpkg.com CDN, which was blocked by the Vite dev server's
Content-Security-Policy. Mocha is loaded as a classic script (UMD, sets
window.mocha) and chai is imported as an ES module and assigned to window.chai.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: julianharty