fix(mqtt): hold newer-version /mqtt_prefs across saves + pin frozen layouts

The unknown-version path kept defaults at boot but any later savePrefs()
(every CLI set command) rewrote /mqtt_prefs as v1 with defaults, destroying
the newer config after a firmware downgrade. Latch _mqtt_prefs_hold when an
unsupported version is seen and refuse to write while it is set — checked
before the NRF52/STM32 open path, which deletes the file first.

Also pin the frozen legacy /mqtt_prefs layouts (472/1464/2904 bytes + 8-byte
header) with static_asserts so every target build re-verifies the deployed
fleet's file offsets, and null-check _obs in AlertReporter::onLoop.

src/helpers/AlertReporter.cpp

@@ -206,7 +206,7 @@ void AlertReporter::formatAge(unsigned long age_ms, char* out, size_t out_size)
 }
 
 void AlertReporter::onLoop(unsigned long now_ms) {
-  if (!_prefs || !_obs->alert_enabled) return;
+  if (!_prefs || !_obs || !_obs->alert_enabled) return;
   if (!_mesh) return;
 
   // Throttle: ~5 s cadence. The thresholds are minutes-scale so this is fine.