feat: implement ViewModels layer with TDD (Milestone 3)

Implemented all ViewModels using strict TDD (RED-GREEN-REFACTOR) with CommunityToolkit.Mvvm.

**ViewModels Implemented:**
- MainViewModel: Coordinates shake detection, estimate generation, and display
- HistoryViewModel: Manages estimate history display and clearing
- SettingsViewModel: Handles app settings (mode selection, history size)

**MainViewModel Features:**
- Subscribes to ShakeDetectionService events
- Generates estimates when shake stops (transition from shaking→not shaking)
- Automatically saves estimates to storage
- Resets shake detection after estimate generation
- Loads and saves settings (SelectedMode)
- Proper disposal pattern (unsubscribe, stop monitoring)

**HistoryViewModel Features:**
- ObservableCollection<EstimateResult> for UI binding
- LoadHistoryCommand to fetch from storage
- ClearHistoryCommand to remove all history
- IsEmpty property for conditional UI display
- Replaces collection contents on reload

**SettingsViewModel Features:**
- SelectedMode property (Work/Generic)
- MaxHistorySize property
- SaveSettingsCommand to persist changes
- Loads settings on initialization

**Tests:**
- MainViewModel: 18 tests (RED-GREEN-REFACTOR)
- HistoryViewModel: 15 tests (RED-GREEN-REFACTOR)
- SettingsViewModel: 13 tests (RED-GREEN-REFACTOR)
- Total: 165 tests, all passing (48 models + 71 services + 46 ViewModels)

**Quality:**
- Build: 0 warnings, 0 errors across all platforms
- All tests use NSubstitute for mocking
- Property change notifications verified
- Async operations properly tested

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/HihaArvio/ViewModels/HistoryViewModel.cs

@@ -0,0 +1,51 @@
+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using HihaArvio.Models;
+using HihaArvio.Services.Interfaces;
+
+namespace HihaArvio.ViewModels;
+
+/// <summary>
+/// ViewModel for displaying and managing estimate history.
+/// </summary>
+public partial class HistoryViewModel : ObservableObject
+{
+    private readonly IStorageService _storageService;
+
+    [ObservableProperty]
+    private ObservableCollection<EstimateResult> _history;
+
+    [ObservableProperty]
+    private bool _isEmpty;
+
+    public HistoryViewModel(IStorageService storageService)
+    {
+        _storageService = storageService;
+        _history = new ObservableCollection<EstimateResult>();
+        _isEmpty = true;
+    }
+
+    [RelayCommand]
+    private async Task LoadHistoryAsync()
+    {
+        var estimates = await _storageService.GetHistoryAsync();
+
+        History.Clear();
+        foreach (var estimate in estimates)
+        {
+            History.Add(estimate);
+        }
+
+        IsEmpty = History.Count == 0;
+    }
+
+    [RelayCommand]
+    private async Task ClearHistoryAsync()
+    {
+        await _storageService.ClearHistoryAsync();
+
+        History.Clear();
+        IsEmpty = true;
+    }
+}