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_st7796.h

@@ -32,6 +32,14 @@ public:
 
     // Backlight control
     void set_backlight(bool on) override;
+    
+    // Brightness control (0-100)
+    void set_brightness(uint8_t brightness) override;
+    uint8_t get_brightness() const override;
+    
+    // Power management
+    void sleep();  // Put display to sleep (low power, touch still active)
+    void wake();   // Wake display from sleep
 
     // Orientation control
     void set_rotation(uint8_t rotation) override;