fix(touch): support placement on touchend without prior touchmove

[Phillipxh]

Summary

• handle touch pointer extraction with changedTouches fallback (not only touches[0])
• extract preview-position calculation into a shared helper
• allow finishAdding(event) to compute preview from pointer event when no prior hover/move was recorded

Why

In mobile flows, placement can happen on touchend without a preceding touchmove. In that case, preview coordinates were not resolved and adding could silently fail.

Tests

• add unit test for getPointerPosition() with touchend + changedTouches
• add unit test proving object placement works on touchend even without prior touchmove

Related

• LibreSign/libresign PR: Fix mobile signature placement flow, save visibility, and coordinate serialization libresign#7342