[web] Display ConnectButton after ConnectModal flow completes successfully

Summary:
Display ConnectButton with wallet info once ConnectModal flow completes successfully and signer (via useSigner() hook) exists.

Depends on D6220

Test Plan:
Here's what it looks like:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6221