Refactor: Migrate MainViewModel to use StateFlow

This commit refactors the `MainViewModel` to use `StateFlow` instead of `mutableStateOf` for managing its observable properties. This change improves the way UI state is handled and observed, aligning with modern Android development best practices.

Additionally, `OnboardingState.kt` has been moved to the `onboarding` package for better organization.

app/src/main/java/com/bitchat/android/onboarding/OnboardingState.kt

@@ -0,0 +1,12 @@
+package com.bitchat.android.onboarding
+
+enum class OnboardingState {
+    CHECKING,
+    BLUETOOTH_CHECK,
+    LOCATION_CHECK,
+    PERMISSION_EXPLANATION,
+    PERMISSION_REQUESTING,
+    INITIALIZING,
+    COMPLETE,
+    ERROR
+}