Refactor: Introduce MainViewModel for onboarding state management

This commit introduces a `MainViewModel` to manage the UI state for the onboarding flow. This change centralizes the onboarding state (including Bluetooth status, location status, error messages, and loading indicators) within the ViewModel, allowing it to survive configuration changes and simplifying state management within `MainActivity`.

Key changes:
- Created `MainViewModel.kt` to hold and manage onboarding-related UI state.
- Moved onboarding state variables (e.g., `onboardingState`, `bluetoothStatus`, `locationStatus`) from `MainActivity` to `MainViewModel`.
- Updated `MainActivity` to observe and update onboarding state through the `MainViewModel`.
- Created `OnboardingState.kt` to define the possible states of the onboarding process.
- Ensured that the onboarding process is not restarted on configuration changes by checking `mainViewModel.onboardingState` before initiating.

app/src/main/java/com/bitchat/android/MainViewModel.kt

@@ -0,0 +1,52 @@
+package com.bitchat.android
+
+import androidx.compose.runtime.*
+import androidx.lifecycle.ViewModel
+import com.bitchat.android.onboarding.BluetoothStatus
+import com.bitchat.android.onboarding.LocationStatus
+
+class MainViewModel : ViewModel() {
+    
+    private var _onboardingState by mutableStateOf(OnboardingState.CHECKING)
+    val onboardingState: OnboardingState get() = _onboardingState
+    
+    private var _bluetoothStatus by mutableStateOf(BluetoothStatus.ENABLED)
+    val bluetoothStatus: BluetoothStatus get() = _bluetoothStatus
+    
+    private var _locationStatus by mutableStateOf(LocationStatus.ENABLED)
+    val locationStatus: LocationStatus get() = _locationStatus
+    
+    private var _errorMessage by mutableStateOf("")
+    val errorMessage: String get() = _errorMessage
+    
+    private var _isBluetoothLoading by mutableStateOf(false)
+    val isBluetoothLoading: Boolean get() = _isBluetoothLoading
+    
+    private var _isLocationLoading by mutableStateOf(false)
+    val isLocationLoading: Boolean get() = _isLocationLoading
+    
+    // Public update functions for MainActivity
+    fun updateOnboardingState(state: OnboardingState) {
+        _onboardingState = state
+    }
+    
+    fun updateBluetoothStatus(status: BluetoothStatus) {
+        _bluetoothStatus = status
+    }
+    
+    fun updateLocationStatus(status: LocationStatus) {
+        _locationStatus = status
+    }
+    
+    fun updateErrorMessage(message: String) {
+        _errorMessage = message
+    }
+    
+    fun updateBluetoothLoading(loading: Boolean) {
+        _isBluetoothLoading = loading
+    }
+    
+    fun updateLocationLoading(loading: Boolean) {
+        _isLocationLoading = loading
+    }
+}