Add account management features and localization updates

[Vishwajeet-11]

Summary

• Adds an Account Core Data entity (MainModel 3) so users can track which account/payment method a transaction came from (cash, credit card, etc.)
• Mirrors category UX: Settings → Accounts for create/edit/delete/reorder
• Adds an optional account picker on the add/edit transaction screen
• Includes account in CSV export; EN/FR strings updated

Closes #107

Test plan

• Open Settings → Accounts and create a few accounts (emoji, name, color)
• Edit / reorder / delete an account (transactions should not be deleted)
• Add a new expense/income and select an account
• Edit an existing transaction and change or clear the account
• Confirm account is optional (can save without selecting one)
• Export data and verify Account column appears in CSV
• Confirm iCloud/existing data still migrates cleanly after model update