- Enable BTN_FWD_GPIO (GPIO 10) as secondary button for mode toggle (Shift+Tab)
- Update BOARD_HAS_SECONDARY_BUTTON flag from 0 to 1
- Clarify button layout: BOOT (primary PTT), PWR (cycle screens via AXP), KEY (secondary mode toggle)
- Document KEY GPIO discovery method (empirical scanning of unused GPIOs)
- Update button_count calculation to include secondary button when available
- Initialize both BTN_BACK_GPIO and BTN_FWD_GPIO in input_hal_init()
- Refactor input_hal_is_held() to use switch statement for cleaner multi-button handling