fix(hub): Hub login must accept the password the desktop accepts

verify_system_password() trusted the first loadable libcrypt. A library that cannot speak the shadow hash's algorithm (yescrypt, NixOS's default) answers with the '*0' sentinel, which was compared to the hash and reported as a wrong password -- even though the same password still signed in at the desktop. Following the System Passwords tile's 'sign out and back in' hint then locked the user out of the Hub, asking for a 'new' Hub password that does not exist: Hub login is the free account password, verified against /etc/shadow.

Try every loadable libcrypt, treat None/'*' replies as inconclusive rather than wrong, and when no library can decide, verify via PAM -- the same check GDM performs. Also replaces the sign-out hint (server + JS fallback) with wording that says the Hub has no separate password and not to sign out unless you know the current desktop password.

Recovery for an already locked-out user whose desktop login still works: on the machine itself, clear the localhost:8937 cookies and open http://localhost:8937/auto-login (localhost-only), or run 'sudo change-free-password' in a terminal to set a known free/Hub password.

app/sovran_systemsos_web/static/js/service-detail.js

@@ -35,7 +35,7 @@ function _renderCredsHtml(credentials, unit) {
       continue;
     }
     if (cred.unknown) {
-      html += '<div class="creds-row"><div class="creds-label">' + escHtml(cred.label) + '</div><div class="creds-value-wrap"><div class="creds-value creds-unknown">' + escHtml(cred.hint || "Changed outside the Hub — sign out and back in to show it here.") + '</div></div></div>';
+      html += '<div class="creds-row"><div class="creds-label">' + escHtml(cred.label) + '</div><div class="creds-value-wrap"><div class="creds-value creds-unknown">' + escHtml(cred.hint || "Changed outside the Hub — the current password is your desktop (free) sign-in password; it will show here after you next sign in to the Hub with it. Do not sign out unless you know that password — the Hub has no separate password.") + '</div></div></div>';
       continue;
     }
     var displayValue = linkify(cred.value);