fix(editor): stop canvas chrome tokens shadowing site custom properties

[DavidBabinec]

The bug

A site that defines a custom property sharing an admin token's name, like :root { --radius: 12px } or its own --text, rendered with the editor's values in the canvas (6px, #ededed) while the published page used the site's. The canvas chrome stylesheet is unlayered and set those admin tokens on the iframe :root, which beats anything in @layer user-authored.

The fix

EditorChromeInjector now forwards every admin token under --chrome-* (--radius becomes --chrome-radius), and the chrome rules read only those. The forwarded set is derived from the rules, so the two cannot drift. Published output is untouched: the injector only runs in the canvas.

New Playwright spec SITE-027 checks the real cascade in Chromium, since happy-dom ignores @layer. createStylesheet moved into the shared E2E helpers.

Verification

bun test                 # 7123 pass, 0 fail (new unit test fails on main)
bunx playwright test --project=e2e canvas-chrome-tokens site-files
                         # SITE-027 and SITE-013 pass; SITE-027 fails on main (6px, expected 12px)
bun run build && bun run lint   # exit 0, exit 0