Central LBS (LED Button Service) sample for nRF Connect SDK.

This application runs as a Bluetooth LE Central and connects to Nordic's
existing `peripheral_lbs` sample.

Implemented behavior:
- Scans for the Nordic LBS service UUID (0x1523)
- Connects to the first matching peripheral
- Discovers the LBS button characteristic (0x1524)
- Discovers the LBS LED characteristic (0x1525)
- Discovers the CCC descriptor for the button characteristic
- Subscribes to button notifications from the peripheral
- Mirrors the peripheral button state onto Central LED0
- Uses Central LED1 as a scan/connection status indicator
- Uses Central Button 0 to write ON/OFF to the peripheral LED characteristic

Central-side behavior:
- LED1 blinks while scanning
- LED1 turns solid ON when connected
- LED1 returns to blinking after disconnect when scanning restarts
- LED0 turns ON when the peripheral button is pressed
- LED0 turns OFF when the peripheral button is released
- Central Button 0 writes to the peripheral LED characteristic:
	- press -> write 1 -> peripheral LED ON
	- release -> write 0 -> peripheral LED OFF

Peripheral-side behavior expected from Nordic `peripheral_lbs`:
- Peripheral Button 0 sends button notifications to the central
- Peripheral LED3 is controlled by writes from the central to the LBS LED
	  characteristic

Build and run:
1. Flash Nordic `peripheral_lbs` on one board.
2. Build and flash this central sample on another board.
3. Open serial output for the central board.
4. Wait for connection and service discovery to complete.
5. Test both directions:
	- press peripheral Button 0 (DK_BTN1) -> central LED0 should follow
	- press central Button 0 (sw0) -> peripheral LED3 should follow

Typical central log sequence:
- "Bluetooth initialized"
- "Scanning started"
- "Found LBS peripheral ... connecting..."
- "Connected"
- "Found LBS service: start ... end ..."
- "Found button characteristic (...)"
- "Found LED characteristic (...)"
- "Discovering descriptors in range ..."
- "Found button CCC descriptor (...)"
- "Subscribed to button notifications"

Typical runtime logs:
- "Button notification: pressed (LED0 ON)"
- "Button notification: released (LED0 OFF)"
- "Central button0 pressed"
- "Central button0 released"
- "Wrote peripheral LED: ON"
- "Wrote peripheral LED: OFF"

Notes:
- This sample is Central/client-side only. It does not host the LBS service.
- The peripheral must provide the LBS service and characteristics.
- The button subscription path depends on successful CCC descriptor discovery.
- Central Button 0 detection is implemented using GPIO polling for robustness on
	  the target board.
