v0.9.3 — AI textarea grow and archive-preserving Backspace

AI rewrite height re-measurement and archive-preserving Backspace deletion.

• After an AI rewrite (or undo/redo and other programmatic text changes), every segment textarea is measured again, so a taller result expands the input instead of overflowing.
• Backspace deleting the last remaining active segment now keeps all archived segments; the fresh empty card is appended instead of replacing the whole note.

AI 优化后文本区自动撑开，Backspace 删除最后一条活动分段不再清空归档。

• AI 优化（以及撤销 / 重做等程序化改写）提交后会重新测量所有分段文本区高度，行数变多时输入框会自动长高。
• Backspace 删除最后一条活动空分段时会保留全部归档分段，只补一个新的空分段。