Splash screen: show unofficial build marker for Plus builds

Add FIRMWARE_PLUS_BUILD compile flag to both companion sections in
platformio.ini. When set, SplashScreen shows "+ unofficial build" in
yellow below the build date. Avoids runtime string parsing — the
existing version stripping logic (cuts at first dash) is unchanged.

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

examples/companion_radio/ui-new/UITask.cpp

@@ -66,6 +66,11 @@ public:
     display.setTextSize(1);
     display.drawTextCentered(display.width()/2, 42, FIRMWARE_BUILD_DATE);
 
+#ifdef FIRMWARE_PLUS_BUILD
+    display.setColor(DisplayDriver::YELLOW);
+    display.drawTextCentered(display.width()/2, 54, "+ unofficial build");
+#endif
+
     return 1000;
   }