fix(fxa-settings): prevent CJK button labels breaking one character per line

[vpomerleau]

Because

• Contributor PR fix(fxa-settings): prevent CJK button labels breaking one character per line #20724 by @sreecharan-desu cannot run CircleCI — this project does not build fork PRs, so the full suite never reports there (see CONTRIBUTING.md). This mirrors the commit onto a branch in this repo so CI runs.
• Settings CTA buttons shrink in flex layouts, so Japanese, Chinese, and Korean labels stack one character per line.

This pull request

• Cherry-picks @sreecharan-desu's commit unmodified onto current main; they remain the commit author.
• .cta-xl now uses grow shrink-0 basis-0 instead of flex-1, so labels are not compressed.
• Adds a higher-specificity shrink-0 whitespace-nowrap rule for the cta-* variants, plus the same for cta-* inside unit-row-actions.
• Lets paired modal action buttons wrap as a group (flex-wrap) when horizontal space is tight.

Issue that this pull request solves

Closes: #18683

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

Screenshots

Before

After

How to review (Optional)

• Key files/areas to focus on: packages/fxa-react/styles/ctas.css, packages/fxa-settings/src/styles/unit-row.css, Settings/Modal/index.tsx, Settings/SubRow/index.tsx
• Suggested review order: shared CTA CSS → unit-row CSS → modal/sub-row JSX
• Risky or complex parts: cta-xl is used 58 times in fxa-settings and 11 call sites combine it with flex-1. The override relies on element+class specificity (button.cta-primary) beating the flex-1 utility. Worth confirming English and German buttons still lay out correctly at mobile and landscape widths.

Other information (Optional)

• No automated coverage is added — the change is CSS and className only. Manual CJK verification is still needed.
• Local run on this branch: fxa-settings integration suite 2948 passed / 27 snapshots passed, tsc --noEmit clean, ESLint clean on both changed TSX files. One failure, Pair2/Supplicant/TimeoutAndCancel, reproduces identically on unmodified main and is unrelated to this change.