Update field names in serializeVisibleElements to use producer & consumer names

[ciaens]

Resolves: #7316

EDIT: only tested on stable32

📝 Summary

In src/store/files.js:868-875, the serializeVisibleElements() function was mapping coordinate fields to the wrong names: x, y, w, h instead of the correct page, top, left, width, height.

The buildVisibleElements() function in VisibleElements.vue creates coordinates with { page, width, height, left, top }, matching what the backend's FileElementService::translateCoordinatesToInternalNotation() expects. But serializeVisibleElements() was looking for x/y/w/h properties that don't exist, so the API received undefined values for all coordinates. This caused the signature position to silently fail to save & no error because the request structure was valid, but the coordinates were empty.

🧪 How to test

• select a file
• add an account for signing
• place a signature somewhere
• check that the signature location just placed is correctly saved by re-opening the doc

✅ Checklist

• I have read and followed the contribution guide.