fix(editor): make manual text OCR-ready

[atirna]

Summary

Manual text now creates the same text-region relationships as detected text. Empty manual text can enter OCR, and its frame has the existing automatic placement with a user override after moving or resizing.

Related issue

Fixes #994

Verification

• cargo test -p koharu-app manual_text_uses_an_editable_ocr_region

AI assistance

I used AI assistance to trace the existing scene relationships and implement this focused change; I reviewed and own the patch.

[github-actions]

Thanks for your first PR to Koharu.

Please review our contribution guide before review:
https://koharu.rs/development/contributing/

In the PR description, include:

• what changed
• any user-visible behavior differences
• how you verified the change

If AI substantially helped design or implement the patch, disclose the extent and purpose of that assistance. A human contributor must personally review, understand, and take responsibility for every submission.

[ZenoArrows]

While the visual style of the text boxes has changed the manually placed text boxes are still not picked up by OCR. Could you verify your change again?