Fix window hover detection; add redaction tool

Window detection was broken by wrong CG-to-AppKit coordinate conversion
of the cursor and highlight rects — both now go through the same
cgToAppKit mapping. The draw toolbar gains a Pen/Redact toggle: redact
drags cover content with a solid black bar (participates in undo/redo
and export like pen strokes).

💘 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>1.5.0</string>
+    <string>1.5.1</string>
     <key>CFBundleVersion</key>
-    <string>50</string>
+    <string>51</string>
     <key>LSUIElement</key>
     <true/>
     <key>NSHighResolutionCapable</key>