Reverse Engineer Lost Firmware or Rewrite Complete Firmware

### Description
The original firmware for the FlightPanel-01 controller is partially lost. The functionality must either be reconstructed through reverse engineering or replaced by a complete new implementation.

The final result must be a clean, maintainable firmware version that fully supports the current hardware configuration.

### Tasks
- Analyze existing hardware design and pin mappings
- Attempt to reconstruct original firmware logic if possible
- If reconstruction is not feasible, implement a complete new firmware
- Support all hardware inputs and outputs:
- Multiplexer inputs
- Direct buttons
- Rotary encoder with push button
- Potentiometers
- LED outputs
- Implement structured and maintainable source code

### Build and Flash Requirements
- Firmware must compile without errors
- Provide a simple build and flash process
- Include a shell script (`flash.sh`) to compile and upload the firmware
- Compatible with ATmega32U4 bootloader

### Testing
- Verify all inputs are detected correctly
- Confirm LED outputs operate as expected
- Validate rotary encoder behavior
- Perform hardware flash test on the board

### Deliverables
- Complete firmware source code
- Build instructions
- `flash.sh` flashing script
- Tested firmware ready for deployment