fix(paywall): respect token decimals in EVM paywall

[Bortlesboat]

Fixes #1979.

Summary

• stop assuming every accepted EVM token uses 6 decimals in the paywall amount display
• read balances and decimals from the actual accepted asset instead of hardcoded USDC helpers
• add regressions for 18-decimal tokens and generic token-balance reads

Verification

• pnpm --dir typescript/packages/http/paywall test -- src/builder.test.ts src/evm/utils.test.ts
• pnpm --dir typescript/packages/http/paywall test
• pnpm --dir typescript/packages/http/paywall exec eslint src/builder.test.ts src/evm/utils.test.ts src/evm/index.ts src/evm/utils.ts src/evm/EvmPaywall.tsx
• pnpm --dir typescript --filter @x402/core --filter @x402/evm --filter @x402/svm --filter @x402/avm build
• pnpm --dir typescript/packages/http/paywall build:paywall

[vercel]

@Bortlesboat is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[phdargen]

Duplicate of #1980