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

CMakeLists.txt

@@ -113,6 +113,7 @@ target_include_directories(basic1 PRIVATE
 target_link_libraries(basic1 
         hardware_spi
         hardware_i2c
+        hardware_pwm
         pico_multicore
         m
 )