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

display/low_level_display.h

@@ -32,6 +32,11 @@ public:
     // Optional: Backlight control (if supported)
     virtual void set_backlight(bool on) { (void)on; }
     
+    // Optional: Brightness control (if supported)
+    // brightness: 0-100 (percentage), 0=off, 100=full brightness
+    virtual void set_brightness(uint8_t brightness) { (void)brightness; }
+    virtual uint8_t get_brightness() const { return 100; }  // Default to full brightness
+    
     // Optional: Orientation control (not commonly needed for bitmap displays)
     virtual void set_rotation(uint8_t rotation) { (void)rotation; }