Show a grey ring behind black filament gauges so they stay visible.

Co-authored-by: Cursor <cursoragent@cursor.com>

esphome/bambu-display/test_dashboard.cpp

@@ -130,6 +130,12 @@ int main() {
   assert(data.active_filament_color() == 0xFF8800);
   data.trays.clear();
   assert(data.active_filament_color() == GREEN);
+  assert(filament_is_black(0x000000));
+  assert(filament_is_black(filament_color("#000000FF")));
+  assert(filament_is_black(0x202020));
+  assert(!filament_is_black(0xFFFFFF));
+  assert(!filament_is_black(GREEN));
+  assert(!filament_is_black(0xFF8800));
 
   FleetCounts counts;
   data.connected = true;