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>

src/HihaArvio/Platforms/Android/MainActivity.cs

@@ -0,0 +1,10 @@
+using Android.App;
+using Android.Content.PM;
+using Android.OS;
+
+namespace HihaArvio;
+
+[Activity(Theme = "@style/Maui.SplashTheme", MainLauncher = true, LaunchMode = LaunchMode.SingleTop, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize | ConfigChanges.Density)]
+public class MainActivity : MauiAppCompatActivity
+{
+}