Fix _initialized never set in Display::init()

Prevents double PSRAM allocation and LVGL driver re-registration
if init() were called more than once.

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

lib/tdeck_ui/Hardware/TDeck/Display.cpp

@@ -70,6 +70,7 @@ bool Display::init() {
         return false;
     }
 
+    _initialized = true;
     INFO("Display initialized successfully");
     return true;
 }