One read of a device's link: state::credentials, with the client's seal

Five places read the server address and token straight from sync_state:
sharing, autosync, sync_unlink, update's download token, and the ffi. Reading it
raw is how Android came to send its sealed token to the share routes (#5381).
state::credentials(conn, seal) now holds that read. With a seal it opens the token
(open_token), and without one (the desktop keeps it plain) it returns it as stored.
Every site calls it.

DRY pass #2, batch 1, F1 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

desktop/src-tauri/src/commands/sync.rs

@@ -136,8 +136,7 @@ pub async fn sync_unlink(db: State<'_, Db>) -> Result<UnlinkResult, String> {
     // the UI.
     let link = {
         let conn = db.conn()?;
-        let current = state::read(&conn).map_err(|e| e.to_string())?;
-        current.server_url.zip(current.device_token)
+        state::credentials(&conn, None).map_err(|e| e.to_string())?
     };
     let revoked = match &link {
         Some((base_url, token)) => client::revoke_self(base_url, token).await,