fix(editor): improve text box editing with the keyboard

[Dronquavious]

Summary

Improve the text-box editing experience, especially when using the software keyboard.

• Reveal a newly created text box above the keyboard while preserving the current page position.
• Keep a trailing blank page available when creating a text box on the last page.
• Give new text boxes a larger default height for multiline writing.
• Enable scrolling within multiline text boxes when their content exceeds the visible area.
• Use iOS keyboard avoidance so the editor remains visible while typing.

Validation

• Rebases cleanly on the latest main
• Uses published @mathnotes/mobile-ink@0.3.3
• npm run typecheck passes