fix(installer): preserve secrets across re-runs

Re-running the installer after a failed build regenerated JWT_SECRET
and MARIADB_ROOT_PASS, overwriting .env with values that didn't match
the already-set MariaDB password. Now reads existing .env values and
only generates new ones on a fresh install.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "nixpanel-server",
-  "version": "0.6.4",
+  "version": "0.6.5",
   "private": true,
   "workspaces": [
     "server",