feat(09-01): add LiveCharts2 NuGet and FileTypeMetric data model

- Add LiveChartsCore.SkiaSharpView.WPF 2.0.0-rc5.4 package reference
- Create FileTypeMetric record with Extension, TotalSizeBytes, FileCount
- Include DisplayLabel computed property for chart label binding

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SharepointToolbox/Core/Models/FileTypeMetric.cs

@@ -0,0 +1,21 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Represents storage consumption for a single file extension across all scanned libraries.
+/// Produced by IStorageService.CollectFileTypeMetricsAsync and consumed by chart bindings.
+/// </summary>
+public record FileTypeMetric(
+    /// <summary>File extension including dot, e.g. ".docx", ".pdf". Empty string for extensionless files.</summary>
+    string Extension,
+    /// <summary>Total size in bytes of all files with this extension.</summary>
+    long TotalSizeBytes,
+    /// <summary>Number of files with this extension.</summary>
+    int FileCount)
+{
+    /// <summary>
+    /// Human-friendly display label: ".docx" becomes "DOCX", empty becomes "No Extension".
+    /// </summary>
+    public string DisplayLabel => string.IsNullOrEmpty(Extension)
+        ? "No Extension"
+        : Extension.TrimStart('.').ToUpperInvariant();
+}