fix: disable code block language selector when editor is not editable

[ysds]

Summary

Fixes #2156

Disable the language selector dropdown in code blocks when editable={false} is set on the BlockNoteView.

Rationale

As reported in #2156, when the editor is set to read-only mode (editable={false}), the code block language selector dropdown remains interactive, allowing users to change the language. This is inconsistent with the expected behavior where all editing functionality should be disabled in read-only mode.

Changes

• Check editor.isEditable before registering the change event listener on the language selector
• Set select.disabled = true when the editor is in read-only mode
• Only register the event listener and cleanup function when the editor is editable

Impact

• No impact on existing functionality when editable={true} (default behavior)
• When editable={false}, the language selector will be visually disabled and non-interactive
• Dynamic changes to editable are properly handled due to Tiptap's setEditable() triggering a re-render of block components

Testing

• Manually tested with editable={false} set on BlockNoteView - language selector is disabled
• Manually tested dynamic toggling of editable - selector state updates correctly when toggling between editable and read-only modes

Screenshots/Video

N/A

Checklist

• [x] Code follows the project's coding standards.
• [ ] Unit tests covering the new feature have been added.
• [x] All existing tests pass.
• [ ] The documentation has been updated to reflect the new feature

Additional Notes