Remove unused navigation, datastore, and hilt-navigation dependencies

[in-jun]

Drops three declared-but-unused dependencies from app/build.gradle.kts and their version-catalog entries. None of them are referenced anywhere in the codebase:

• androidx.navigation:navigation-compose — navigation is a hand-rolled when (screen) in MainActivity; there is no NavHost/NavController.
• androidx.datastore:datastore-preferences — persistence goes through SharedPreferences/EncryptedSharedPreferences/Room; no DataStore usage exists.
• androidx.hilt:hilt-navigation-compose — the ViewModel is obtained via by viewModels(), not hiltViewModel().

Also removes the now-orphaned navigationCompose and datastore version entries. The hiltNavigationCompose version ref is kept since androidx-hilt-work and androidx-hilt-compiler still reference it.

:app:compileDebugKotlin and :core-sync:compileKotlin both pass.

Fixes #43

[in-jun]

Verified each removal against the source tree:

• navigation-compose — no NavHost/NavController/rememberNavController anywhere; navigation is the hand-rolled when (screen) in MainActivity. Safe.
• datastore-preferences — no DataStore references; persistence is SharedPreferences/EncryptedSharedPreferences/Room. Safe.
• hilt-navigation-compose — the ViewModel is retrieved via by viewModels() (MainActivity.kt:42), and the @HiltViewModel annotation resolves from dagger.hilt.android.lifecycle (hilt-android), not this artifact. Safe.

Version-catalog cleanup checks out too: navigationCompose and datastore were each only referenced by the removed aliases, while hiltNavigationCompose is correctly kept since androidx-hilt-work and androidx-hilt-compiler still point at it. No dangling references remain and there's no runtime surface affected.

Looks good.