Firefox MV3 compatibility (background scripts + gecko settings)

[lhero-org]

Addresses the Firefox Add-ons (AMO) item on the roadmap — the next post-1.0 target, flagged there as needing "a manifest v2/v3 compatibility audit; Firefox's WebExtension surface diverges from Chromium in a few places."

Changes:

• manifest.json: adds background.scripts alongside service_worker — Chrome uses service_worker, Firefox (121+) uses scripts as an event page. Both keys can coexist safely per current Firefox/Chrome behavior.
• manifest.json: adds browser_specific_settings.gecko for AMO signing. Note: the id value (browser-extension@wordpress.org) is a placeholder — this needs to be a real, permanent identifier the WordPress Foundation controls, since AMO uses it to track the extension across updates. Please confirm the actual value before merge.
• background.js: guards the importScripts() calls, since importScripts is a WorkerGlobalScope method unavailable in Firefox's event-page context (a plain script, not a worker). Firefox loads the two dependency files via manifest.json's scripts array instead, ahead of background.js.
• Safari resource mirror (safari/.../Resources/) synced to match, via the existing scripts/sync-safari.sh.

Tested:

• Full test suite passes (test/npm test), npm run build clean.
• Manually verified in Firefox desktop on Linux, loaded as a temporary add-on (about:debugging): WordPress detection, popup, admin bar shortcuts (edit/view/profile/logout), Mobile Preview, cache-busting, Clear Site Data (Keep Login), and Highlight Blocks all confirmed working correctly.

Known gap, not addressed here: Firefox for Android has no windows API, so Mobile Preview (which calls chrome.windows.create) will silently no-op there rather than open a preview window. Scoping that out of this PR since it's Android-specific and this PR is focused on desktop Firefox compatibility — happy to follow up separately if that's wanted.