refactor: Migrate to zero-dependency wagmi wallet adapter and remove RainbowKit

[kushbosamiya]

Remove RainbowKit and replace with native wagmi wallet layer

Problem

@rainbow-me/rainbowkit (v2.2.8) is currently the wallet layer for this app — wired in through providers/WalletProvider.tsx and importing third-party styles into app/globals.css. It's doing more than needed and taking control away from us in the places where we need it most: unrecognized chains, stale cached sessions, and connection state edge cases.

Beyond the bloat, this directly conflicts with the zero-dependency stance already in place across the ecosystem. Both Fate-EVM-Frontend and OrbOracle-EVM-Frontend have already dropped RainbowKit in favor of custom wagmi/viem connection flows. This repo is the odd one out.

What needs to change

• Uninstall @rainbow-me/rainbowkit and remove the stylesheet import from globals.css
• Tear down the RainbowKitProvider wrapper and the theme injection in WalletProvider.tsx
• Replace with a useWalletConnection hook using useAccount, useConnect, and useDisconnect directly
• Port the ConnectBtn and ChainSwitcher components over from Fate-EVM-Frontend — these already handle network verification at runtime

Acceptance criteria

• @rainbow-me/rainbowkit fully removed from package.json and the lockfile
• Connect, disconnect, and chain switching all work through native wagmi hooks
• No regressions in signature/transaction submission across existing forms
• npm run dev and npm run build pass with zero hydration warnings or missing-style errors