fix: block auto-save when DDNS availability check is unreachable

'unreachable' should not be a terminal state that triggers auto-save —
it was causing a 503 when the availability check failed and auto-save
fired the backend registration attempt. Only 'available' allows
auto-save when the cell name has changed from the loaded value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/Settings.jsx

@@ -691,10 +691,10 @@ function Settings() {
     if (!identityDirty) return;
     if (ipRangeError || cellNameError || domainError) return;
     // In pic_ngo mode, if the cell name differs from what was last saved/loaded,
-    // wait for the availability check to reach a terminal state before saving.
-    // 'available' and 'unreachable' are terminal; null/'checking'/'taken' are not.
+    // only auto-save once the check confirms the name is available.
+    // All other states (null, 'checking', 'taken', 'unreachable') block auto-save.
     if (domainMode === 'pic_ngo' && identity.cell_name !== loadedCellName) {
-      if (picAvail !== 'available' && picAvail !== 'unreachable') return;
+      if (picAvail !== 'available') return;
     }
     const timer = setTimeout(() => saveIdentityRef.current(), 800);
     return () => clearTimeout(timer);