feat: Add dbDirectory parameter for custom database storage locations

[nightfall708]

Summary

• Adds a dbDirectory parameter to PowerSyncDatabase() that allows specifying a custom directory path for the database file
• Enables storing the database in a shared App Group container, which is essential for iOS apps that need to share data between the main app and extensions (e.g. Share Extensions, widgets)
• The underlying Kotlin SDK already supports dbDirectory — this change threads it through the Swift API layer

Motivation

Currently, dbFilename only accepts a filename (not a path), and the database is always stored in the hardcoded Application Support directory. This makes it impossible to share the database between the main app and app extensions without going through a cloud round-trip.

Before: Share Extension writes to its own PowerSync database → syncs to cloud → main app syncs from cloud (slow, requires connectivity)

After: Both main app and Share Extension point to the same database file in the App Group container (instant local access)

Usage

let containerURL = FileManager.default.containerURL(
    forSecurityApplicationGroupIdentifier: "group.com.example.app"
)

let db = PowerSyncDatabase(
    schema: schema,
    dbFilename: "powersync.db",
    dbDirectory: containerURL?.path
)

Changes

File	Change
PowerSyncDatabase.swift	Add dbDirectory: String? = nil parameter to the public API
KotlinPowerSyncDatabaseImpl.swift	Thread dbDirectory through to the Kotlin SDK; use it in deleteDatabase()
KotlinPowerSyncDatabaseImplTests.swift	Add tests for custom directory creation, file location, and deletion
CHANGELOG.md	Document the new feature

Test plan

• Verify build compiles (swift build ✅)
• Verify tests compile (swift build --build-tests ✅)
• testCustomDbDirectory — verifies database is created in custom directory, not the default
• testCustomDbDirectoryCloseWithDeleteDatabase — verifies all SQLite files (db, wal, shm) are cleaned up from custom directory
• Existing tests continue to pass (default behavior unchanged when dbDirectory is nil)