Add optional sync between Macs with a shared 8-digit code

Notes and photos were stuck on one machine. A small Go/Postgres backend
stores each group's payload under a code that doubles as the shared
secret, so a second Mac just needs the code — no account or login.

Applying a remote change must not push it straight back, otherwise the
two Macs would bump the version back and forth forever, so payloads are
fingerprinted and identical ones are skipped.

💘 Generated with Crush

Assisted-by: Crush:kimi-k3

build-app.sh

@@ -49,6 +49,14 @@ cat > "$APP/Contents/Info.plist" <<PLIST
     <true/>
     <key>NSPrincipalClass</key>
     <string>NSApplication</string>
+    <!-- The sync server URL is user-supplied and may be plain HTTP (a LAN
+         address or a self-hosted box), which App Transport Security would
+         otherwise block. -->
+    <key>NSAppTransportSecurity</key>
+    <dict>
+        <key>NSAllowsArbitraryLoads</key>
+        <true/>
+    </dict>
 </dict>
 </plist>
 PLIST