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.cpp

@@ -69,9 +69,24 @@ void LowLevelDisplayST7796::refresh() {
 }
 
 void LowLevelDisplayST7796::set_backlight(bool on) {
-    // ST7796 driver doesn't have backlight control yet
-    // TODO: Add GPIO control for backlight pin
-    (void)on;
+    // Use brightness control: on = 100%, off = 0%
+    st7796_set_brightness(on ? 100 : 0);
+}
+
+void LowLevelDisplayST7796::set_brightness(uint8_t brightness) {
+    st7796_set_brightness(brightness);
+}
+
+uint8_t LowLevelDisplayST7796::get_brightness() const {
+    return st7796_get_brightness();
+}
+
+void LowLevelDisplayST7796::sleep() {
+    st7796_sleep();
+}
+
+void LowLevelDisplayST7796::wake() {
+    st7796_wake();
 }
 
 void LowLevelDisplayST7796::set_rotation(uint8_t rotation) {