Check Accessibility before starting a capture; log capture geometry

The first capture used to start the overlay and the system permission
prompt at once, making both uninteractive. begin() now requires
Accessibility up front, shows an alert with a direct link to the setting,
and only starts capturing once granted. Captures log their display,
backing scale, and rect mapping for diagnosing multi-display offsets.

💘 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>1.7.0</string>
+    <string>1.7.1</string>
     <key>CFBundleVersion</key>
-    <string>70</string>
+    <string>71</string>
     <key>LSUIElement</key>
     <true/>
     <key>NSHighResolutionCapable</key>