feat(explorer): add MPP voucher receipt page

Adds /receipt/voucher route that renders offchain payment channel state
from a signed MPP voucher. Accepts channelId, cumulativeAmount, and
signature as URL search params.

- Reads channel state on-chain via the stream channel escrow contract
- Verifies voucher signature via EIP-712 typed data recovery
- Shows on-chain state (deposit, settled) alongside offchain state
  (cumulative amount, unsettled delta, remaining balance)
- Displays channel status (open/closing/closed) and participants
- Unverified signatures show a warning banner
- noindex/nofollow to prevent voucher URL leakage via search engines

Co-Authored-By: Georgios Konstantopoulos <17802178+gakonst@users.noreply.github.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019d9db9-63d7-70bb-be99-3c3693f8db6b

Author: decofe