fix: gate Fast Transfer by source-chain support

[UfukNode]

After #160, the app opens with Arc Testnet as the source and Fast Transfer selected. Circle currently lists Arc as a Standard Transfer source only, so the default UI promises a faster route that is not available for that source. The same mismatch applies to several other supported source chains.

This change:

• keeps the Fast tab disabled for source chains that do not support it
• defaults Arc to Standard and resets the selection when switching from a Fast-capable source
• validates the capability again in the transfer function so the rule does not depend on UI state
• selects the fee entry with finality threshold 1000 instead of assuming the first API item is the Fast fee

Fast-capable sources follow Circle's supported chains table:
https://developers.circle.com/cctp/concepts/supported-chains-and-domains

Checked with:

• npm run lint (passes; existing getBalance hook warning remains)
• npm run build
• browser flow: Arc -> Arbitrum -> select Fast -> Arc; Fast is re-enabled for Arbitrum and the selection returns to Standard for Arc

[UfukNode]

@hjchen-circle, #160 correctly made the old RPC rewrite in #149 unnecessary. While checking the current Arc flow, I found that Arc is now the default source with Fast selected even though it is Standard-only as a source in Circle's support table. This PR updates that current behavior and keeps the check in both the UI and transfer path. I verified the Arc/Arbitrum/Arc selection flow in a browser as well as lint and production build.