Add Decodex app usage estimates

[yvette-carlisle]

Summary

• add bounded seven-day account usage history and pool-level usage estimates
• surface Pool used, Day delta, Daily avg, and per-account 7d usage in the macOS app
• keep account operations on the bundled Rust helper and document the local usage-history file

Validation

• git diff --check
• cargo make test-decodex-app-stage
• cargo test -p decodex usage --all-features
• curl /api/accounts smoke confirmed usage_estimate and per-account seven-day data
• semantic drift audit reviewed: docs claims match the executable usage-history and app-helper surfaces