fix(document): account for page padding in rotation layer sizing

[kduncanhsu]

Summary

• Fixes bug where annotations on PDF's appear in slightly different positions when PDF is rotated
• Solution: subtract the 30px vertical page padding (15px top + 15px bottom from Preview SDK) when computing annotation layer dimensions
• pageEl.clientHeight was being used directly, but it includes padding that isn't part of the annotation content area — causing the layer to be oversized and percentage-based annotation coordinates to resolve to slightly wrong pixel positions

Test plan

• Draw and save annotations on a PDF at 0° rotation
• Rotate the PDF 90°, 180°, and 270° — verify annotations remain precisely positioned
• Verify annotations near page edges (where offset was most visible) are no longer shifted

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ kduncanhsu
❌ Duncan Hsu

---

Duncan Hsu seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}