feat(boards): Add secondary button support to Waveshare ESP32-C6-Touch-AMOLED-2.16

- 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

firmware/src/boards/waveshare_amoled_216_c6/input.cpp

@@ -2,16 +2,17 @@
 #include "board.h"
 #include <Arduino.h>
 
-// Only BOOT is wired to a MCU GPIO. The "PWR" side button is the AXP2101
-// PKEY, handled in power.cpp.
-
 void input_hal_init(void) {
     pinMode(BTN_BACK_GPIO, INPUT_PULLUP);
+    pinMode(BTN_FWD_GPIO,  INPUT_PULLUP);
 }
 
 bool input_hal_is_held(InputButton btn) {
-    if (btn == INPUT_BTN_PRIMARY) {
+    switch (btn) {
+    case INPUT_BTN_PRIMARY:
         return digitalRead(BTN_BACK_GPIO) == LOW;
+    case INPUT_BTN_SECONDARY:
+        return digitalRead(BTN_FWD_GPIO) == LOW;
     }
     return false;
 }