CP-11621 - Add opaque background option to signature box

[bernardodsanderson]

CP-11621 - Add opaque background option to signature box

What

Adds an "Add opaque background" / "Remove opaque background" option to the signature field config menu in the template builder. When enabled, the signed signature renders on an opaque white background in the signing view, the completed submission page, and the final PDF, so signatures stay visible when the uploaded page surface is dark.

Why

Captured during a testing party: transparent signature PNGs disappear against dark page surfaces. Builders need a per-field way to force an opaque white box behind the signature.

How to test

1. Open a template with a signature field in the builder, open the field config menu, and toggle "Add opaque background" (text swaps to "Remove opaque background" when on). Save and reload to confirm it persists.
2. As a signer, complete the signature on a dark-surface document and confirm the signature shows on a white box in the signing overlay.
3. Open the completed submission and confirm the white background behind the signature.
4. Download/generate the result PDF and confirm the signature sits on a white rectangle.

Screenshots

https://careerplug.atlassian.net/browse/CP-11621

[jewls618]

No issues found 👍