Command Block UI Overhaul

Fixed Issues

• After setValue truncation, layout still used the original long text
• A single insert/delete triggered the responder multiple times, and the cursor briefly jumped to the end of the full text in between
• Reducing the maximum length did not rebuild lines or the cursor
• Word deletion used UTF-16 length but converted to code point offsets
• Unicode text truncation could split surrogate pairs
• Mouse positioning ignored the top padding and lacked drag selection
• Multiple usage/error messages used the same Y coordinate
• The completion list had no screen boundary handling, and scrolling to the last item was offset
• Old asynchronous completion requests could refresh new input, and callbacks were not thread-confined
• After suggestions were closed, late requests could still reopen them
• Enter-to-save did not check whether Done was available; resizing directly enabled controls
• Subsequent command block packets reinitialized the edited content
• Toggling the output switch directly modified the client-side block object
• The multi-line container used LinkedList but frequently accessed by index

New Features

• Undo/redo: 100 bounded snapshots restoring text, cursor, and selection; new edits clear the redo branch.
• Mouse multi-line drag selection and edge scrolling.
• Window resizing preserves editing state and undo history.