Support message signing

[devanoneth]

What is the feature about

Message signing was removed in the forking process due to scope. It should be added back in / tested.

The list of requirements

• Support message signing as the current official web app does.

Designs/sketches

Follow the same UI as the official web app.

Links

n/a

[chainflo-agent]

Tracking follow-up from PR #73 safe-apps restore: AppFrame now intentionally throws for SDK off-chain signature lookup () when the signature is not already in local state. We need to re-implement off-chain signature retrieval once this issue is resolved so Safe Apps can fetch prepared signatures without Gateway fallback.

[chainflo-agent]

Follow-up (clarified): in Safe Apps AppFrame we now intentionally throw for SDK off-chain signature lookup (getOffChainSignature) when signature is not already in local state. TODO is in code to re-implement this once issue #7 is closed so Safe Apps can fetch prepared signatures again without Gateway fallback.