Implement Rich Text

Drawy's current text tool is limited to plain text with no per-character formatting, no undo/redo, no support for non-Latin input methods, and performance issues on large text. The goal of this project is to refactor and extend the text tool into a rich text editor. The implementation will begin by splitting the existing code into three classes (TextItem, TextCursor, TextTool) and replacing the manual rendering logic with QTextLayout, which natively handles BiDi, line breaking, and cursor positioning. On top of this foundation, the project will deliver per-range character formatting, text alignment, word wrapping, spell checking via KDE Sonnet, and a formatting toolbar for the user to control fonts, styles, and alignment. Undo/redo will be integrated. Save/load will be extended to support rich text while remaining backward compatible with existing documents. Finally, the backing data structure will be replaced with a Gap Buffer for O(1) insertions and deletions.