docs: add USDC payments dApp guide (BasePay on Base Mainnet)

[osr21]

Summary

This guide shows developers how to build a production-grade USDC payments dApp on Base Mainnet — covering the full stack from wallet connection through on-chain smart contract interactions.

Reference implementation: BasePay — live on Base Mainnet
Source code: github.com/osr21/basepay-dapp

What the guide covers

• Wallet connection with wagmi v2 + viem on Base Mainnet
• Direct USDC transfer and router-based sends with atomic 0.30% fee deduction
• Batch payments to up to 200 recipients in a single transaction (BatchPay contract)
• Shareable payment request links + QR codes
• Trustless USDC escrow with timeout refund (Escrow contract)
• Recurring pull-payment subscriptions (SubscriptionManager contract)
• Security checklist: bounded approvals, CEI pattern, pause mechanism, BaseScan verification

Contracts deployed on Base Mainnet (source-verified)

Contract	Address
BasePayRouter	0x2d7ba7ed34f8fa16fe4d0d11b51306dc753812c8
BatchPay	0x82569caf7847040a03ad2c6545ade5af2bdcf47c
Escrow	0x5b3241a47acfda41f15dfd7260339e2a88d52318
SubscriptionManager	0x546093b0476b4b7909cd84f3a0fef813c421d14a

Changes

• docs/apps/guides/usdc-payments-dapp.mdx — new guide page
• docs/docs.json — added apps/guides/usdc-payments-dapp to the Apps > Guides navigation group

Fits naturally alongside the existing "base-notifications" technical guide and "migrate-to-standard-web-app" guide in the Apps section.

[cb-heimdall]

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[osr21]

check github page for documentation about dapp, some fixes applied, but overall need some feedback