fix: camera — null-check sensor handle before set_vflip/set_hmirror

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

firmware/src/camera.cpp

@@ -54,8 +54,10 @@ bool camera_init() {
 
     // Flip vertically — adjust if mounting orientation differs
     sensor_t* s = esp_camera_sensor_get();
+    if (s) {
         s->set_vflip(s, 1);
         s->set_hmirror(s, 0);
+    }
 
     return true;
 }