Add startup guards and UI diagnostic logging

[rygel]

Summary

• add a cross-process monitor launch mutex in MonitorLauncher to prevent duplicate monitor starts and wait for existing startup
• add a per-user Slim UI single-instance mutex in App.xaml.cs with clean acquire/release logging
• add persistent UI diagnostic log writer to %LOCALAPPDATA%\\AIUsageTracker\\logs with 14-day retention pruning
• route MainWindow diagnostic events to both structured logger and file diagnostics
• add runtime reliability TODO items for multi-instance and monitor duplicate-start hardening

Testing

• dotnet test AIUsageTracker.Tests/AIUsageTracker.Tests.csproj --filter "FullyQualifiedName~MonitorStartupPathTests|FullyQualifiedName~MonitorResilienceTests"
• dotnet test AIUsageTracker.Tests/AIUsageTracker.Tests.csproj --filter "FullyQualifiedName~AppStartupTests|FullyQualifiedName~MainWindow"

[github-actions]

Build Performance Report

⏱️ Build Time: 12 minutes

📊 Comparison with main branch:

• Baseline (avg): 10 minutes
• Current: 12 minutes
• Change: 📈 20%

✅ Within acceptable range

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This is an automated performance check