feat: Initial support for the x3 (#875)

## Summary

Adds Xteink X3 hardware support to CrossPoint Reader. The X3 uses the
same SSD1677 e-ink controller as the X4 but with a different panel
(792x528 vs 800x480), different button layout, and an I2C fuel gauge
(BQ27220) instead of ADC-based battery reading.

All X3-specific behavior is gated by runtime device detection — X4
behavior is unchanged.

Depends on community-sdk X3 support: open-x4-epaper/community-sdk#19
(merged).

## Changes

### HAL Layer

**HalGPIO** (`lib/hal/HalGPIO.cpp/.h`)
- I2C-based device fingerprinting at boot: probes for BQ27220 fuel
gauge, DS3231 RTC, and QMI8658 IMU to distinguish X3 from X4
- Detection result cached in NVS for fast subsequent boots
- Exposes `deviceIsX3()` / `deviceIsX4()` helpers used throughout the
codebase
- X3 button mapping (7 GPIOs vs X4's layout)
- USB connection detection and wake classification for X3

**HalDisplay** (`lib/hal/HalDisplay.cpp/.h`)
- Calls `einkDisplay.setDisplayX3()` before init when X3 is detected
- Requests display resync after power button / flash wake events
- Runtime display dimension accessors (`getDisplayWidth()`,
`getDisplayHeight()`, `getBufferSize()`)
- Exposed as global `display` instance for use by image converters

**HalPowerManager** (`lib/hal/HalPowerManager.cpp/.h`)
- X3 battery reading via I2C fuel gauge (BQ27220 at 0x55, SOC register)
- X3 power button uses GPIO hold for deep sleep

### Display & Rendering

**GfxRenderer** (`lib/GfxRenderer/GfxRenderer.cpp/.h`)
- Buffer size and display dimensions are now runtime values (not
compile-time constants) to support both panel sizes
- X3 anti-aliasing tuning: only the darker grayscale level is applied to
avoid washed-out text on the X3 panel. X4 retains both levels via
`deviceIsX4()` gate

**Image Converters** (`lib/JpegToBmpConverter`, `lib/PngToBmpConverter`)
- Cover image prescale target uses runtime display dimensions from HAL
instead of hardcoded 800x480

### UI Themes

**BaseTheme / LyraTheme** (`src/components/themes/`)
- X3 button position mapping for the different physical layout
- Adjusted UI element positioning for 792x528 viewport

### Boot & Init

**main.cpp**
- X3 hardware detection logging
- Adjusted init sequence for X3 (no `HalSystem::begin()` dependency on
X3 path)

**HomeActivity**
- Uses runtime `renderer.getBufferSize()` instead of static
`GfxRenderer::getBufferSize()`

FYI I did not add support for the gyro page turner. That can be it's own
PR.

lib/hal/HalGPIO.h

@@ -1,7 +1,6 @@
 #pragma once
 
 #include <Arduino.h>
-#include <BatteryMonitor.h>
 #include <InputManager.h>
 
 // Display SPI pins (custom pins for XteinkX4, not hardware SPI defaults)
@@ -18,6 +17,27 @@
 
 #define UART0_RXD 20  // Used for USB connection detection
 
+// Xteink X3 Hardware
+#define X3_I2C_SDA 20
+#define X3_I2C_SCL 0
+#define X3_I2C_FREQ 400000
+
+// TI BQ27220 Fuel gauge I2C
+#define I2C_ADDR_BQ27220 0x55  // Fuel gauge I2C address
+#define BQ27220_SOC_REG 0x2C   // StateOfCharge() command code (%)
+#define BQ27220_CUR_REG 0x0C   // Current() command code (signed mA)
+#define BQ27220_VOLT_REG 0x08  // Voltage() command code (mV)
+
+// Analog DS3231 RTC I2C
+#define I2C_ADDR_DS3231 0x68  // RTC I2C address
+#define DS3231_SEC_REG 0x00   // Seconds command code (BCD)
+
+// QST QMI8658 IMU I2C
+#define I2C_ADDR_QMI8658 0x6B        // IMU I2C address
+#define I2C_ADDR_QMI8658_ALT 0x6A    // IMU I2C fallback address
+#define QMI8658_WHO_AM_I_REG 0x00    // WHO_AM_I command code
+#define QMI8658_WHO_AM_I_VALUE 0x05  // WHO_AM_I expected value
+
 class HalGPIO {
 #if CROSSPOINT_EMULATED == 0
   InputManager inputMgr;
@@ -26,9 +46,19 @@ class HalGPIO {
   bool lastUsbConnected = false;
   bool usbStateChanged = false;
 
+ public:
+  enum class DeviceType : uint8_t { X4, X3 };
+
+ private:
+  DeviceType _deviceType = DeviceType::X4;
+
  public:
   HalGPIO() = default;
 
+  // Inline device type helpers for cleaner downstream checks
+  inline bool deviceIsX3() const { return _deviceType == DeviceType::X3; }
+  inline bool deviceIsX4() const { return _deviceType == DeviceType::X4; }
+
   // Start button GPIO and setup SPI for screen and SD card
   void begin();
 
@@ -41,6 +71,14 @@ class HalGPIO {
   bool wasAnyReleased() const;
   unsigned long getHeldTime() const;
 
+  // Setup wake up GPIO and enter deep sleep
+  void startDeepSleep();
+
+  // Verify power button was held long enough after wakeup.
+  // If verification fails, enters deep sleep and does not return.
+  // Should only be called when wakeup reason is PowerButton.
+  void verifyPowerButtonWakeup(uint16_t requiredDurationMs, bool shortPressAllowed);
+
   // Check if USB is connected
   bool isUsbConnected() const;
 
@@ -61,4 +99,4 @@ class HalGPIO {
   static constexpr uint8_t BTN_POWER = 6;
 };
 
-extern HalGPIO gpio;  // Singleton
+extern HalGPIO gpio;