feat: add Xteink X3 hardware support

Adds HAL-level support for the Xteink X3 (SSD1677 controller, 792x528
display). Includes device detection, display initialization, button
mapping, power management, and theme adjustments for the X3 form factor.

lib/hal/HalDisplay.h

@@ -49,6 +49,14 @@ class HalDisplay {
 
   void displayGrayBuffer(bool turnOffScreen = false);
 
+  // Runtime geometry passthrough
+  uint16_t getDisplayWidth() const;
+  uint16_t getDisplayHeight() const;
+  uint16_t getDisplayWidthBytes() const;
+  uint32_t getBufferSize() const;
+
  private:
   EInkDisplay einkDisplay;
 };
+
+extern HalDisplay display;