fix(addon): make font url() rewrite quote-aware to prevent parsable-css crash

[aaspinwall]

What changed?

Made the webfont url() rewrite in packages/addon/scripts/build.sh quote-aware, and added a build-time guard.

• The rewrite that converts absolute url(/fonts/...) to relative url(../fonts/...) previously used s{url\(/fonts/}{url(../fonts/}g, which only matched the unquoted form. It now captures an optional quote ((["\x27]?)) and re-emits it, so url(/fonts/, url('/fonts/, and url("/fonts/ are all rewritten.
• Added a guard that greps the bundled CSS after the rewrite and fails the build (printing the offending lines) if any absolute /fonts/ url() survived, so a broken bundle can never silently ship again.

I verified the new regex against the actual deployed CSS: all 36 url('/fonts/...') references convert to url('../fonts/...'), and the guard passes on the rewritten output (and fails on the pre-fix input).

AI disclosure: this change was diagnosed and written by Claude (agent), reviewed and directed by the author. The author identified the failing try run; the agent traced the root cause from the CI log, wrote the one-line regex fix and guard, and verified the rewrite against the deployed CSS.

Why?

A try push crashed in Thunderbird's static browser_parsable_css.js check:

TEST-UNEXPECTED-CRASH | comm/mail/base/test/browser/static/browser_parsable_css.js
PROCESS-CRASH | Missing chrome or resource URLs:
  resource://builtin-addons/fonts/Inter/Inter-Regular.woff2 [@ mozilla::net::CheckForBrokenChromeURL]

The bundled CSS shipped absolute url('/fonts/...') font references. Loaded as a built-in add-on, an absolute /fonts/... URL resolves against the resource:// protocol root (resource://builtin-addons/fonts/...) rather than the add-on's own directory, so the font is missing and the static check fatally crashes (Bug 2036665).

build.sh already had a rewrite step meant to prevent exactly this, but its regex only matched unquoted url(/fonts/. Production (minified) builds strip quotes so the bug stayed hidden; the dev/unminified build that was deployed preserved the source single quotes (url('/fonts/...')), so the rewrite was a no-op and the absolute paths shipped — crashing CI.

Failing run: https://treeherder.mozilla.org/jobs?repo=try-comm-central&revision=51fc152ffe02dc4a5ec85cedbb188463172bb7f2&selectedTaskRun=dHZ5CqI4QRSnRU3g-hftrg.0

Limitations and Notes

• The font files themselves were always packaged correctly; only the CSS URL form was wrong.
• Follow-up (process, not code): the deployed .xpi was a dev/unminified build, which is what exposed the gap. Standardizing the deploy/release path on the production pnpm build would add defense in depth, though this fix makes the rewrite correct for both modes.
• Still needs a fresh .xpi rebuild + re-deploy into the comm tree and a new try push to confirm the suite goes green.

Applicable Issues

Closes #941

Screenshots

N/A — build-script change.