Add touch reader controls for page navigation

Implements tap zones on touch devices for page back/forward navigation and press-and-hold actions, mirroring physical button behavior. Adds isXteinkDevice() helper to distinguish Xteink X3/X4 boards from touch devices. The sunlight fading fix setting is now exclusive to Xteink devices, while touch devices get the new touch reader controls toggle instead.

lib/hal/HalGPIO.cpp

@@ -251,6 +251,11 @@ unsigned long HalGPIO::lastTouchHeldMs() const { return inputMgr.lastTouchHeldMs
 
 bool HalGPIO::hasTouch() const { return inputMgr.hasTouch(); }
 
+bool HalGPIO::isXteinkDevice() const {
+  const auto board = BoardConfig::ACTIVE.board;
+  return board == BoardConfig::Board::XteinkX3 || board == BoardConfig::Board::XteinkX4;
+}
+
 void HalGPIO::startDeepSleep() {
   // Ensure that the power button has been released to avoid immediately turning back on if you're holding it
   while (inputMgr.isPressed(BTN_POWER)) {

lib/hal/HalGPIO.h

@@ -59,6 +59,13 @@ class HalGPIO {
   inline bool deviceIsX3() const { return _deviceType == DeviceType::X3; }
   inline bool deviceIsX4() const { return _deviceType == DeviceType::X4; }
 
+  // True on the Xteink X3/X4 transflective C3 boards. Distinct from
+  // deviceIsX3/X4 (which only tell the two C3 variants apart and both stay
+  // "X4" on non-C3 boards): this keys off BoardConfig::ACTIVE.board, so it is
+  // the reliable "is this an Xteink device" check used to gate features that
+  // are Xteink-only (sunlight fading fix) or non-Xteink-only (touch controls).
+  bool isXteinkDevice() const;
+
   // Start button GPIO and setup SPI for screen and SD card
   void begin();