fix: canonicalize SOL amounts as lamports

[newmattock]

What changed

Related to #1. This PR makes the SOL amount path canonical and fail-closed:

• Adds a strict decimal-SOL parser with exact lamport conversion.
• Rejects scientific notation, zero/negative values, surrounding whitespace, sub-lamport precision, non-canonical serialized lamports, and unsafe transaction ranges.
• Uses the same lamport integer for policy caps, daily accounting, approval fingerprints, receipts, and SystemProgram.transfer.
• Validates non-blocked Solana receipts and documents the serialized representation while preserving verification of legacy v1 amountSol receipts.
• Adds deterministic parser, arithmetic, policy, receipt, and UI-safe display coverage.

The blocked invalid-amount receipt path is preserved and rendered as Invalid amount instead of crashing the local app.

Validation

• npm test -- --maxWorkers=1 --no-file-parallelism (38 passed)
• npm run lint
• npm run build
• Added deterministic tests for exact conversion, rejection boundaries, integer arithmetic, invalid policy input, and receipt binding.
• Smoke-tested the Vite preview: a sub-lamport input is blocked and the receipt remains visible.
• No layout change; the visible changes are amount formatting and the safe invalid-receipt label.

Safety

• No private keys, seed phrases, wallet connections, or network transactions were requested.
• Policy evaluation still occurs before any wallet signing request.
• No dependencies, generated build output, or secrets were added.

AI assistance

This contribution was implemented by an AI coding agent. I manually verified the source diff, the required validation commands, and the local browser smoke test.