Explain the SSL failure instead of just reporting it

A Postgres URL without an sslmode made lib/pq ask for TLS and die with
"SSL is not enabled on the server", which reads like a server fault when
it is really a connection-string default. The error now says what to
append, and the server retries briefly so a deploy is not marked
unhealthy just because Postgres was still starting.

The sync server URL also defaults to the hosted instance now, so a fresh
install needs no setup.

README.md

@@ -62,15 +62,16 @@ Or build the Docker image (`docker build -t prompt-sync Server/`) and point it
 at your own Postgres — see [Server/README.md](Server/README.md) for that and for
 deploying to a host.
 
-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.
+Then in the panel: click the sync icon (or Cmd+,), confirm the server URL, 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.
-  See [Server/README.md](Server/README.md) to deploy it on a VPS or home server.
+- Sync defaults to `https://prompt.tzgyn.dev`; change it in the sync panel to
+  point at your own server. Both Macs must be able to reach it — a LAN address
+  or a Tailscale hostname works, and `localhost` only works if the server runs
+  on the same Mac. See [Server/README.md](Server/README.md) to deploy it.
 - Joining a group that already has notes replaces what's on that Mac.
 - The app polls every 5 seconds (a tiny version check) and pushes 1.5s after a
   change; last write wins, and changes that came from the server are not echoed