Replace RainbowKit with Reown AppKit

[dmytrotkk]

RainbowKit is in maintenance mode - last feature release was mid-2025, no EIP-5792 support, no plans for it. Reown AppKit (formerly Web3Modal) is actively developed, and gives us things we need: native EIP-5792 sendCalls support, defineChain for custom SKALE chains, one-click SIWE auth, built-in analytics, and 600+ wallet support via WalletConnect v2.

This ticket swaps the wallet connection layer only. The underlying wagmi/viem stays the same.

What changes

• Install @reown/appkit + @reown/appkit-adapter-wagmi
• Remove @rainbow-me/rainbowkit
• Get Reown Cloud project ID (free tier)
• Replace RainbowKitProvider with createAppKit() initialization
• Replace <ConnectButton /> with <appkit-button />
• Configure all SKALE app-chains via defineChain using existing viem chain definitions (Europa, Calypso, Nebula, Titan + Mainnet)
• Disable social logins: socials: false
• Auto-provision SKALE chains on wallet connect (wallet_addEthereumChain for all hubs on connect)
• Refactor read operations to use direct PublicClient per chain - only switch wallet chain when signing

Reown provides an official migration guide: https://docs.reown.com/appkit/migration/from-rainbowkit-next

Acceptance criteria

• Wallet connect/disconnect works with MetaMask, Coinbase Wallet, and WalletConnect mobile
• All SKALE chains appear in network selector and can be added to wallet
• Social login options are hidden
• No RainbowKit imports remain
• Read operations don't trigger chain switch popups
• Chain switches only happen when user signs on a different chain