Respect textarea editability before applying formatting

[feritcemkahraman]

Formatting buttons can change a readonly or disabled textarea, including one disabled by its surrounding fieldset. This also emits an input event even though the field is not editable.

Check the field's readonly and native disabled states before applying formatting. The regressions verify that restricted fields keep their value and emit no input event, then format normally again when enabled.

Validation: all three regressions failed before the fix; all 147 Chromium tests now pass through the repository's Karma runner. The TypeScript distribution/declaration build and full configured ESLint check pass using the installed lockfile dependencies on Node 24. The Windows cleanup wrapper was bypassed by running the same tools directly; Firefox and WebKit were not run.

[Copilot]

🟢 Approval recommended

The focused guard and regression tests cover the intended restrictions and recovery behavior without unresolved issues.

No open issues 🎉

What changed in this PR

Prevents toolbar formatting from modifying textareas that are not editable.

Changes:

• Guard formatting for readonly, disabled, and fieldset-disabled textareas.
• Add regressions verifying unchanged values, no input events, and normal formatting after re-enabling.

File	Description
test/​test.js	Tests all three restrictions and recovery.
src/​index.ts	Checks editability before applying formatting.

_{Review effort: Max}