Make exported stroke thickness match the preview

Strokes were sized in view points but exported in image pixels, so on
retina captures the saved/copied annotation looked several times thinner.
Stroke widths are now converted into image pixels when drawn and scaled
back for the preview, keeping preview and export identical.

💘 Generated with Crush

Assisted-by: Crush:deepseek-v4-flash-0731

build-app.sh

@@ -108,9 +108,9 @@ cat > "$APP/Contents/Info.plist" <<'PLIST'
     <key>CFBundleIconFile</key>
     <string>Icon</string>
     <key>CFBundleShortVersionString</key>
-    <string>1.9.1</string>
+    <string>1.9.2</string>
     <key>CFBundleVersion</key>
-    <string>91</string>
+    <string>92</string>
     <key>LSUIElement</key>
     <true/>
     <key>NSHighResolutionCapable</key>