feat(mesh-payment): offline Solana payments through beacon relay

[Magicred-1]

Summary

• New executeMeshPayment() in src/services/meshExecutePayment.ts — orchestrates offline Solana payments through a beacon relay node
• Fetch durable nonce via mesh RPC (no internet required), derive ATAs for payer/recipient/broadcaster, partially sign the execute_payment tx with the payer key, zero the key immediately, send partial tx to beacon over LXMF
• Beacon co-signs with broadcaster key and submits to Solana — payer key never leaves the device, no key material sent over mesh
• Fix duplicate rpcAdapter params in sendTransaction.ts destructuring (builds were succeeding but was confusing noise)

Flow

Payer device                    Beacon relay
   |                                |
   |-- mesh RPC: getAccountInfo --> |-- forward to Solana RPC
   |<-- nonce account data ------   |
   |                                |
   | [derive ATAs locally]          |
   | [partialSign tx, zero key]     |
   |                                |
   |-- LXMF: { type, partialTx } -> |-- broadcaster signs + submits
                                    |-- Solana confirms

Test plan

• Payment flow compiles with npx tsc --noEmit
• Unit: executeMeshPayment throws when nonce account not found
• Unit: secret key buffer is zeroed before partialSignExecutePayment call
• Integration: partial tx reaches beacon, beacon submits, tx confirmed on devnet