fix: config_save_wifi — always write both credentials

Replace short-circuit boolean evaluation of putString return values with
separate size_t variables so both writes always execute regardless of
whether the first succeeds.

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

firmware/src/config.cpp

@@ -25,10 +25,10 @@ bool config_load(DeviceConfig& cfg) {
 bool config_save_wifi(const String& ssid, const String& pass) {
     Preferences prefs;
     prefs.begin(NS, false);
-    bool ok = prefs.putString("wifi_ssid", ssid) &&
-              prefs.putString("wifi_pass", pass);
+    size_t r1 = prefs.putString("wifi_ssid", ssid);
+    size_t r2 = prefs.putString("wifi_pass", pass);
     prefs.end();
-    return ok;
+    return (r1 > 0) && (r2 > 0);
 }
 
 bool config_has_wifi() {