Make stroke thickness per-stroke; restore Clear button

Each drawn stroke now stores its own width and color, so changing the
thickness slider no longer retroactively resizes existing strokes. The
Clear button is back in the draw toolbar.

💘 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>1.4.1</string>
+    <string>1.4.2</string>
     <key>CFBundleVersion</key>
-    <string>41</string>
+    <string>42</string>
     <key>LSUIElement</key>
     <true/>
     <key>NSHighResolutionCapable</key>