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.

src/activities/home/HomeActivity.cpp

@@ -138,7 +138,7 @@ bool HomeActivity::storeCoverBuffer() {
   // Free any existing buffer first
   freeCoverBuffer();
 
-  const size_t bufferSize = GfxRenderer::getBufferSize();
+  const size_t bufferSize = renderer.getBufferSize();
   coverBuffer = static_cast<uint8_t*>(malloc(bufferSize));
   if (!coverBuffer) {
     return false;
@@ -158,7 +158,7 @@ bool HomeActivity::restoreCoverBuffer() {
     return false;
   }
 
-  const size_t bufferSize = GfxRenderer::getBufferSize();
+  const size_t bufferSize = renderer.getBufferSize();
   memcpy(frameBuffer, coverBuffer, bufferSize);
   return true;
 }