Add auto-sleep/wake functionality with timer-based dimming

- Added PWM brightness control to ST7796 driver (0-100%)
- Implemented hardware sleep mode for ST7796 (saves ~150mA)
- Added sleep/wake functions preserving framebuffer and settings
- Implemented timer-based inactivity detection (checks every 10s)
- Auto-sleep after 5 minutes of no user input
- Touch controller remains active during TFT sleep for instant wake
- E-ink display also sleeps after timeout, requires re-init on wake
- Added hardware_pwm library dependency to CMakeLists.txt
- Brightness and sleep/wake methods exposed through display abstraction layer

lib/st7796/st7796.h

@@ -357,6 +357,93 @@ void st7796_draw_triangle(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1, ui
  */
 void st7796_fill_triangle(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1, uint16_t x2, uint16_t y2, uint16_t color);
 
+/**
+ * @brief Set display brightness using PWM on backlight pin
+ * 
+ * Controls the backlight LED brightness using hardware PWM. The brightness
+ * is specified as a percentage (0-100), where:
+ * - 0 = Backlight completely off (minimum brightness)
+ * - 100 = Backlight fully on (maximum brightness)
+ * - Values in between = Proportional PWM duty cycle
+ * 
+ * PWM Configuration:
+ * - Frequency: 1000 Hz (1 kHz) - high enough to avoid flicker
+ * - Resolution: 16-bit (0-65535 range internally)
+ * - Smooth transitions without visible flickering
+ * 
+ * Use cases:
+ * - Power saving: Reduce brightness when idle
+ * - Auto-dimming: Lower brightness after timeout
+ * - Manual control: User brightness preferences
+ * - Night mode: Very low brightness for dark environments
+ * 
+ * @param brightness Brightness level (0-100 percent)
+ *                   0 = off, 100 = full brightness
+ * 
+ * Performance: Instant - PWM is handled by hardware
+ * 
+ * Note: Must be called after st7796_init() which configures the backlight pin
+ */
+void st7796_set_brightness(uint8_t brightness);
+
+/**
+ * @brief Get current display brightness level
+ * 
+ * Returns the currently configured brightness level as a percentage (0-100).
+ * This reflects the last value set by st7796_set_brightness() or the
+ * default value (100) if brightness was never explicitly set.
+ * 
+ * @return Current brightness level (0-100 percent)
+ */
+uint8_t st7796_get_brightness(void);
+
+/**
+ * @brief Put display into sleep mode (low power)
+ * 
+ * Enters sleep mode to save power while keeping the controller initialized.
+ * In sleep mode:
+ * - Display is turned off (blank screen)
+ * - Internal oscillator is stopped
+ * - Framebuffer contents are preserved
+ * - Backlight is turned off (PWM set to 0)
+ * - Power consumption is minimized (~10μA typical)
+ * 
+ * Touch controller remains active and functional since it's on a separate
+ * I2C bus. Touch interrupts can wake the system from sleep.
+ * 
+ * Use st7796_wake() to exit sleep mode and restore display.
+ * 
+ * Commands sent:
+ * - DISPOFF (0x28): Turn off display
+ * - SLPIN (0x10): Enter sleep mode
+ * - Backlight PWM set to 0
+ * 
+ * Performance: ~120ms to enter sleep mode
+ */
+void st7796_sleep(void);
+
+/**
+ * @brief Wake display from sleep mode
+ * 
+ * Exits sleep mode and restores display to normal operation.
+ * After waking:
+ * - Internal oscillator restarts
+ * - Display controller becomes active
+ * - Framebuffer contents are preserved
+ * - Display is turned on
+ * - Backlight is restored to previous brightness
+ * 
+ * Commands sent:
+ * - SLPOUT (0x11): Exit sleep mode
+ * - DISPON (0x29): Turn on display
+ * - Backlight PWM restored to saved level
+ * 
+ * Performance: ~120ms to fully wake up
+ * 
+ * Note: Must call st7796_init() before first use of sleep/wake
+ */
+void st7796_wake(void);
+
 #ifdef __cplusplus
 }
 #endif