[FEATURE]: -Set up Drift database with local data models

[4555jan]

Feature and its Use Cases

Set up Drift as the offline database layer for Zplit,
with all core data models defined as Drift table classes.

• All group, transaction, balance, and wallet data persists locally on device
• User opens app without internet — data still loads from local DB

Additional Context

Tables

• UsersTable
• GroupsTable
• TransactionsTable
• BalancesTable

Code of Conduct

• I have joined the Discord server and will post updates there
• I have searched existing issues to avoid duplicates