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

.gitignore

@@ -5,6 +5,9 @@
 # App bundle (build artifact, regenerate with ./build-app.sh)
 Prompt.app/
 
+# Go server binary (go build ./... writes it next to the source)
+Server/server
+
 # Secrets
 *.p8