[web] Lift RainbowKitProvider "above" LoginForm

Summary: In subsequent diff will separate out SIWE button and SIWEForm so we need to lift up RainbotKitProvider so it's available "throughout" LoginForm.

Test Plan:

1. SIWE flow continues to work as expected
2. In subsequent diff, both SIWEButton and SIWEForm component are able to access what they need from RainbowKitProvider.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6488