feat(ui): add click-to-enlarge Mermaid diagrams with zoom overlay and export

Click any Mermaid diagram in chat to open a full-size overlay with:
- Pinch-to-zoom and drag-to-pan (200% default zoom)
- Zoom controls with percentage display
- Export menu: PNG, SVG, or copy raw Mermaid source
- High-resolution re-render at 2x for crisp detail
- Escape or click backdrop to dismiss

Also adds MermaidWebRenderer.renderToSVG() for extracting SVG
markup directly from the mermaid.js render pipeline.

Bumps version to 20260411.1.

Author: fewtarius

Info.plist

@@ -19,9 +19,9 @@
 	<key>CFBundlePackageType</key>
 	<string>APPL</string>
 	<key>CFBundleShortVersionString</key>
-	<string>20260409.1</string>
+	<string>20260411.1</string>
 	<key>CFBundleVersion</key>
-	<string>20260409.1</string>
+	<string>20260411.1</string>
 	<key>LSApplicationCategoryType</key>
 	<string>public.app-category.productivity</string>
 	<key>LSMinimumSystemVersion</key>

Resources/whats-new.json

@@ -1,5 +1,32 @@
 {
   "releases": [
+    {
+      "version": "20260411.1",
+      "release_date": "April 11, 2026",
+      "introduction": "This release adds interactive Mermaid diagrams - click any diagram to open a zoomable, pannable full-size viewer with high-resolution rendering and PNG export.",
+      "improvements": [
+        {
+          "id": "diagram-zoom-overlay",
+          "icon": "arrow.up.left.and.arrow.down.right",
+          "title": "Click-to-Enlarge Mermaid Diagrams",
+          "description": "Mermaid diagrams in conversations are now interactive. Click any diagram to open a full-size overlay with pinch-to-zoom, click-and-drag panning, and zoom controls. Diagrams re-render at double resolution for crisp detail when zoomed in. A small expand icon in the corner hints at the interaction."
+        },
+        {
+          "id": "diagram-png-export",
+          "icon": "square.and.arrow.down",
+          "title": "Export Diagrams as PNG, SVG, or Mermaid Source",
+          "description": "The diagram zoom overlay includes an export menu with three options: save as PNG for raster images, save as SVG for scalable vector graphics editable in tools like Inkscape or Draw.io, and copy the raw Mermaid source to your clipboard for pasting into other tools."
+        }
+      ],
+      "bugfixes": [
+        {
+          "id": "streaming-completion-vanish",
+          "icon": "arrow.down.circle.fill",
+          "title": "Fixed Content Vanishing on Streaming Completion",
+          "description": "Fixed a regression where conversation content could disappear when a streaming response finished. This was a follow-up fix to the scroll system rewrite."
+        }
+      ]
+    },
     {
       "version": "20260409.1",
       "release_date": "April 9, 2026",

Sources/UserInterface/Chat/CachedDiagramView.swift

@@ -17,17 +17,48 @@ struct CachedDiagramView: View {
 
     @State private var isRendering = false
     @State private var renderError: String?
+    @State private var showingZoomOverlay = false
 
     var body: some View {
         Group {
             if let cachedImage = cache[mermaidCode] {
                 // Use cached image - instant display
-                Image(nsImage: cachedImage)
-                    .resizable()
-                    .interpolation(.high)
-                    .aspectRatio(contentMode: .fit)
-                    .frame(maxWidth: min(cachedImage.size.width, 560))
-                    .padding(.vertical, 4)
+                ZStack(alignment: .topTrailing) {
+                    Image(nsImage: cachedImage)
+                        .resizable()
+                        .interpolation(.high)
+                        .aspectRatio(contentMode: .fit)
+                        .frame(maxWidth: min(cachedImage.size.width, 560))
+                        .padding(.vertical, 4)
+
+                    // Zoom hint icon
+                    Image(systemName: "arrow.up.left.and.arrow.down.right")
+                        .font(.system(size: 10, weight: .semibold))
+                        .foregroundColor(.secondary)
+                        .padding(5)
+                        .background(.ultraThinMaterial)
+                        .cornerRadius(4)
+                        .padding(8)
+                        .opacity(0.7)
+                }
+                .onTapGesture {
+                    showingZoomOverlay = true
+                }
+                .onHover { hovering in
+                    if hovering {
+                        NSCursor.pointingHand.push()
+                    } else {
+                        NSCursor.pop()
+                    }
+                }
+                .sheet(isPresented: $showingZoomOverlay) {
+                    DiagramZoomOverlay(
+                        mermaidCode: mermaidCode,
+                        initialImage: cachedImage,
+                        isPresented: $showingZoomOverlay
+                    )
+                    .frame(minWidth: 800, minHeight: 600)
+                }
             } else if let error = renderError {
                 errorView(error)
             } else {