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/HalPowerManager.cpp

@@ -11,7 +11,15 @@
 HalPowerManager powerManager;  // Singleton instance
 
 void HalPowerManager::begin() {
-  pinMode(BAT_GPIO0, INPUT);
+  if (gpio.deviceIsX3()) {
+    // X3 uses an I2C fuel gauge for battery monitoring.
+    // I2C init must come AFTER gpio.begin() so early hardware detection/probes are finished.
+    Wire.begin(X3_I2C_SDA, X3_I2C_SCL, X3_I2C_FREQ);
+    Wire.setTimeOut(4);
+    _batteryUseI2C = true;
+  } else {
+    pinMode(BAT_GPIO0, INPUT);
+  }
   normalFreq = getCpuFrequencyMhz();
   modeMutex = xSemaphoreCreateMutex();
   assert(modeMutex != nullptr);
@@ -78,8 +86,35 @@ void HalPowerManager::startDeepSleep(HalGPIO& gpio) const {
 }
 
 uint16_t HalPowerManager::getBatteryPercentage() const {
+  if (_batteryUseI2C) {
+    const unsigned long now = millis();
+    if (_batteryLastPollMs != 0 && (now - _batteryLastPollMs) < BATTERY_POLL_MS) {
+      return _batteryCachedPercent;
+    }
+
+    // Read SOC directly from I2C fuel gauge (16-bit LE register).
+    // On I2C error, keep last known value to avoid UI jitter/slowdowns.
+    Wire.beginTransmission(I2C_ADDR_BQ27220);
+    Wire.write(BQ27220_SOC_REG);
+    if (Wire.endTransmission(false) != 0) {
+      _batteryLastPollMs = now;
+      return _batteryCachedPercent;
+    }
+    Wire.requestFrom(I2C_ADDR_BQ27220, (uint8_t)2);
+    if (Wire.available() < 2) {
+      _batteryLastPollMs = now;
+      return _batteryCachedPercent;
+    }
+    const uint8_t lo = Wire.read();
+    const uint8_t hi = Wire.read();
+    const uint16_t soc = (hi << 8) | lo;
+    _batteryCachedPercent = soc > 100 ? 100 : soc;
+    _batteryLastPollMs = now;
+    return _batteryCachedPercent;
+  }
   static const BatteryMonitor battery = BatteryMonitor(BAT_GPIO0);
-  return battery.readPercentage();
+  _batteryCachedPercent = battery.readPercentage();
+  return _batteryCachedPercent;
 }
 
 HalPowerManager::Lock::Lock() {