Make Wallet transfer gas defaults privacy-aware

[HDauven]

Summary

• Makes transfer gas defaults privacy-aware: public/Moonlight transfers default to 2_000_000, shielded/Phoenix transfers default to 15_000_000.
• Keeps existing higher defaults for shield, unshield, stake, unstake, withdraw reward, and contract calls unchanged.
• Updates wallet send UI max-fee/default gas calculations to use the recipient privacy rail.
• Keeps execution watcher timeout/errors as pending/unknown instead of marking submitted txs failed. Actual executed events with errors still mark txs failed.
• Updates provider/agent docs and tests for the new defaults and pending-tx lifecycle rule.

Context

Testnet Phoenix transfers are currently failing with Unspendable(OutOfGas) around the old generic transfer default. Current testnet observations show Phoenix transfers spending about 10,913,765 gas without memo and 11,688,779 with a small memo, so 15_000_000 leaves headroom while reducing the public transfer default from the old generic 10_000_000.

Phoenix spends can also remain in node mempool after the wallet execution watcher times out. A watcher timeout is not proof of transaction failure, and mislabeling it as failed encourages users to retry/bump incorrectly while the same spend/nullifier may still be reserved.

Validation

• npm ci
• npm run test:run
• npm run build:chrome
• npm run build:firefox

Notes

This PR only updates wallet-side defaults and timeout labeling. It does not change node mempool DEFAULT_MIN_GAS_LIMIT, implement replace-by-fee semantics, or add a manual pending-nullifier cleanup path for stale Phoenix reservations.