docs(readme): add USDT Payment example and fix Transaction args decimal scaling

[samuelchimmy]

What changed

README.md — one commit, two targeted fixes

Fix 1: Transaction example args decimal clarification

The existing Transaction example passes args: ["0x717...", 1] — a raw uint256 value of 1. For cUSD (18 decimals) this is 0.000000000000000001 cUSD, not 1 cUSD. This is misleading for any developer copying the example.

Changed to 1_000_000_000_000_000_000n (1 cUSD in wei) and added inline comments explaining:

• How to scale for cUSD (18 decimals)
• How to scale for USDT (0x48065fbBE25f71C9282ddf5e1cD6D6A887483D5e, 6 decimals → 1_000_000n)

Fix 2: Add USDT Payment example

The Payment section has one example using cUSD. The Identity props table already lists token: "CELO" | "cUSD" | "USDT" showing USDT is a supported token type, but there is no Payment example showing how to use it.

Added a ### Example with USDT subsection immediately after the existing PaymentBasic example with:

• USDT mainnet address as a named constant with a decimal comment
• Complete working PaymentUSDT component following the exact same pattern as PaymentBasic
• A one-line explanation that amount is passed as a human-readable string

Why

The Transaction example with args: [address, 1] is the kind of mistake that causes real fund loss — a developer copying it and substituting their own token address would send a transaction that appears to succeed but transfers a negligible amount. The corrected value makes the scaling requirement explicit.

The USDT Payment example closes a gap between the props documentation (which mentions USDT) and the usage examples (which only show cUSD).

How to verify

1. Search for 1_000_000_000_000_000_000n in README.md — should appear in the Transaction example with accompanying decimal comments
2. Search for PaymentUSDT — the complete component should appear under ### Example with USDT
3. Search for 48065fbBE — USDT address should appear in both the Transaction comment and the PaymentUSDT constant
4. USDT address verified at celoscan.io/token/0x48065fbBE25f71C9282ddf5e1cD6D6A887483D5e