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

README.md

@@ -12,6 +12,8 @@ to your work. Click anything to copy it back to your clipboard.
   (or Shift+Cmd+C) for a numbered list ready to paste into a chat.
 - **Merge Notes, Mark as Done, Edit, Move To** — right-click any item.
 - **Search** — Cmd+F searches across all sections.
+- **Sync (optional)** — generate an 8-digit code (`1234-5678`) on one Mac and
+  enter it on the other; no account, no login. The code is the only secret.
 - **Local & private** — everything lives in
   `~/Library/Application Support/Prompt/` (`store.json` + `Images/`). No
   account, no sync, no telemetry.
@@ -42,6 +44,32 @@ brew trust TZGyn/tap
 brew install --cask prompt
 ```
 
+## Sync server
+
+Sync is optional and needs the small Go backend in `Server/`. There is no
+authentication: a sync group is identified by an 8-digit code, and whoever has
+the code can read and write the group.
+
+```bash
+createdb prompt_sync
+cd Server
+PROMPT_SYNC_DATABASE_URL="postgres://localhost:5432/prompt_sync?sslmode=disable" \
+  go run .          # listens on :8080
+```
+
+Then in the panel: click the sync icon, set the server URL (e.g.
+`http://192.168.1.10:8080` or an HTTPS host), and either **Generate Code** on
+the first Mac or paste the code into **Join** on the other.
+
+Notes:
+
+- Both Macs must be able to reach the server. A LAN address or a Tailscale
+  hostname works; `localhost` only works if the server runs on the same Mac.
+- Joining a group that already has notes replaces what's on that Mac.
+- The app polls every 5 seconds and pushes 1.5s after a change; last write
+  wins. Changes that came from the server are not echoed back.
+- Verify sync from a terminal with `Prompt --sync-check`.
+
 ## Build from source
 
 ```bash