feat: project setup and core models with TDD

MILESTONE 1 COMPLETE:

Project Setup:
- Create .NET 8 MAUI solution targeting iOS, macOS, and web
- Configure nullable reference types and warnings as errors
- Add required dependencies (MAUI 8.0.3, SQLite, CommunityToolkit.Mvvm)
- Add testing dependencies (xUnit, NSubstitute, FluentAssertions, Coverlet)
- Create comprehensive .editorconfig with C# coding standards
- Update CLAUDE.md with development commands

Core Models (TDD - 48 tests, all passing):
- EstimateMode enum (Work, Generic, Humorous modes)
- EstimateResult model with validation (intensity 0.0-1.0, non-null text)
- ShakeData model with intensity validation
- AppSettings model with defaults (Work mode, max history 10)

Build Status:
- All platforms build successfully (net8.0, iOS, macOS)
- 0 warnings, 0 errors
- Test coverage: 51.28% line, 87.5% branch (models have ~100% coverage)

Per spec: Strict TDD followed (RED-GREEN-REFACTOR), models fully validated

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

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

tests/HihaArvio.Tests/Models/AppSettingsTests.cs

@@ -0,0 +1,90 @@
+using FluentAssertions;
+using HihaArvio.Models;
+
+namespace HihaArvio.Tests.Models;
+
+public class AppSettingsTests
+{
+    [Fact]
+    public void AppSettings_DefaultConstructor_ShouldSetWorkModeAsDefault()
+    {
+        // Act
+        var settings = new AppSettings();
+
+        // Assert
+        settings.SelectedMode.Should().Be(EstimateMode.Work);
+    }
+
+    [Fact]
+    public void AppSettings_DefaultConstructor_ShouldSetMaxHistorySizeTo10()
+    {
+        // Act
+        var settings = new AppSettings();
+
+        // Assert
+        settings.MaxHistorySize.Should().Be(10);
+    }
+
+    [Theory]
+    [InlineData(EstimateMode.Work)]
+    [InlineData(EstimateMode.Generic)]
+    [InlineData(EstimateMode.Humorous)]
+    public void AppSettings_ShouldAllowModeChanges(EstimateMode mode)
+    {
+        // Arrange
+        var settings = new AppSettings();
+
+        // Act
+        settings.SelectedMode = mode;
+
+        // Assert
+        settings.SelectedMode.Should().Be(mode);
+    }
+
+    [Theory]
+    [InlineData(1)]
+    [InlineData(5)]
+    [InlineData(10)]
+    [InlineData(50)]
+    [InlineData(100)]
+    public void AppSettings_ShouldAcceptValidMaxHistorySizeValues(int size)
+    {
+        // Arrange
+        var settings = new AppSettings();
+
+        // Act
+        settings.MaxHistorySize = size;
+
+        // Assert
+        settings.MaxHistorySize.Should().Be(size);
+    }
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-1)]
+    [InlineData(-10)]
+    public void AppSettings_ShouldThrowForInvalidMaxHistorySize(int invalidSize)
+    {
+        // Arrange
+        var settings = new AppSettings();
+
+        // Act
+        Action act = () => settings.MaxHistorySize = invalidSize;
+
+        // Assert
+        act.Should().Throw<ArgumentOutOfRangeException>()
+            .WithMessage("*must be greater than 0*");
+    }
+
+    [Fact]
+    public void AppSettings_ShouldCreateWithDefaultValues()
+    {
+        // Act
+        var settings = new AppSettings();
+
+        // Assert
+        settings.Should().NotBeNull();
+        settings.SelectedMode.Should().Be(EstimateMode.Work);
+        settings.MaxHistorySize.Should().Be(10);
+    }
+}