Inherit the editor font in the highlight overlay

[Ciinz-04]

Makes the highlight overlay inherit the editor font so the caret stays aligned in Safari on macOS, and adds a browser test that compares the font metrics of the overlay with the textarea's.

Tested on macOS 27.0 with Safari 27.0 (22625.1.29.11.27). The drift was there from the first lines I typed, grew with the line count, and did not go away after starting a new session. After running document.querySelector("#editor-highlight code").style.font = "inherit" in the developer console, the caret stayed aligned with the text on every line.

Refs #82

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Summary by cubic

Fixes the highlight overlay caret drifting away from the text on macOS by making the overlay inherit the editor font instead of the user-agent monospace.

• Adds a browser test that compares the overlay's font metrics with the textarea's across all platforms, not just font family.

Refs #82.

^{Written for commit 0fb42c3. Summary will update on new commits.}

[ColtenOuO]

A few notes on the description and commit message:

1. The commit message says "On macOS that is a different font from the textarea's ui-monospace". As far as I know, only Safari resolves ui-monospace; Chrome and Firefox skip it and use a later entry in the stack. That would explain why #82 saw no drift in Chrome on the same Mac, so "Safari on macOS" would be more precise.

2. In #82, jserv asked whether the drift persists while typing or is temporary and goes away after a new session, as in #155. A font mismatch should show up from the first keystroke and should not go away on its own. Could you say in the description which one you saw? If #155's case isn't explained by this change, Refs #82 may fit better than Closes #82 for now, or note that the temporary case is still open.

[Phonlin]

The CSS fix looks right to me. Agree with jserv's comments that the test should run in CI and compare the full font metrics, and with the point about Closes vs Refs #82 given #155.

[Ciinz-04]

The commit message now says Safari on macOS, since Chrome and Firefox skip ui-monospace.

The drift I saw was there from the first lines I typed, grew with the line count, and did not go away after starting a new session. A font mismatch cannot explain a drift that clears on its own as in #155, so the description now says Refs #82 instead of Closes #82.

The check moved to tests/browser/editor-pipeline.test.js and compares fontFamily, fontSize, lineHeight, fontWeight and letterSpacing of #editor and #editor-highlight code; the browser-check.cjs change is dropped. On main it fails on fontFamily (monospace against the ui-monospace stack), it passes with the fix, and adding a font-size to #editor-highlight code makes it fail again.

One correction: the offline lane does run in CI, as the browser_check_accepts_running_rust_server_offline_interview step in check.yml, though not under scripts/test.sh.

[ColtenOuO]

LGTM, great work.