Move whiteboard persistence to file storage

Switch whiteboard note storage from legacy DB rows to per-note JSON files and add migration support. Update WhiteboardNoteSnapshot schema (version bump, viewport, canvas, expires, PathSvgData) and change IWhiteboardNotePersistenceService.SaveNote to return bool to surface write failures (e.g. read-only files). Implement file-based WhiteboardNotePersistenceService with legacy DB migration/cleanup, retention handling, and logging. Add comprehensive unit tests for persistence, stroke path builder, SVG import and viewport helper. Also add ThirdParty/DotNetCampus.InkCanvas project and reference it in the main csproj, and bump PostHog package to 2.6.0.

LanMountainDesktop.Tests/WhiteboardStrokePathBuilderTests.cs

@@ -0,0 +1,77 @@
+using DotNetCampus.Inking.Primitive;
+using LanMountainDesktop.Views.Components;
+using Xunit;
+
+namespace LanMountainDesktop.Tests;
+
+public sealed class WhiteboardStrokePathBuilderTests
+{
+    [Fact]
+    public void BuildPath_WithEmptyPointList_ReturnsEmptyPath()
+    {
+        using var path = WhiteboardStrokePathBuilder.BuildPath(Array.Empty<InkStylusPoint>(), inkThickness: 3d);
+
+        Assert.True(path.IsEmpty);
+    }
+
+    [Fact]
+    public void BuildPath_WithSinglePoint_CreatesVisibleStroke()
+    {
+        using var path = WhiteboardStrokePathBuilder.BuildPath(
+            [CreatePoint(24, 32)],
+            inkThickness: 6d);
+
+        Assert.False(path.IsEmpty);
+        Assert.True(path.Bounds.Width >= 5.5f);
+        Assert.True(path.Bounds.Height >= 5.5f);
+    }
+
+    [Fact]
+    public void BuildPath_WithMultiplePoints_CreatesFilledStroke()
+    {
+        using var path = WhiteboardStrokePathBuilder.BuildPath(
+            [
+                CreatePoint(10, 10),
+                CreatePoint(30, 18),
+                CreatePoint(52, 14)
+            ],
+            inkThickness: 4d);
+
+        Assert.False(path.IsEmpty);
+        Assert.True(path.Bounds.Width > 40f);
+        Assert.True(path.Bounds.Height > 4f);
+    }
+
+    [Fact]
+    public void BuildPath_WithThickerStroke_ExpandsStrokeBounds()
+    {
+        var points = new[]
+        {
+            CreatePoint(10, 10),
+            CreatePoint(80, 10)
+        };
+
+        using var thinPath = WhiteboardStrokePathBuilder.BuildPath(points, inkThickness: 1d);
+        using var thickPath = WhiteboardStrokePathBuilder.BuildPath(points, inkThickness: 8d);
+
+        Assert.True(thickPath.Bounds.Height > thinPath.Bounds.Height);
+    }
+
+    [Fact]
+    public void BuildPath_WithNonFinitePoints_UsesRemainingFinitePoints()
+    {
+        using var path = WhiteboardStrokePathBuilder.BuildPath(
+            [
+                CreatePoint(double.NaN, 10),
+                CreatePoint(20, 20)
+            ],
+            inkThickness: 4d);
+
+        Assert.False(path.IsEmpty);
+    }
+
+    private static InkStylusPoint CreatePoint(double x, double y)
+    {
+        return new InkStylusPoint(x, y, pressure: 1f);
+    }
+}