Prioritize frequently used slash commands

[aaronflorey]

Summary

• track slash command usage locally per app install so ranking works across all projects on that device
• rank slash command autocomplete results by usage before the existing above-input reversal so the most-used match stays nearest the composer
• add focused tests for slash command parsing, ranking, and usage persistence while keeping the change scoped to the app client

Testing

• npm run test --workspace=@getpaseo/app -- src/utils/slash-command-usage.test.ts src/stores/slash-command-usage-store.test.ts