buffyboard: add sticky_shift config option

Adds a keyboard config option to control shift key behavior.
When sticky_shift is enabled (default), the keyboard remains
in uppercase mode until the shift key is pressed again.
When disabled, the keyboard switches back into lowercase mode
when a non-modifier key is pressed.

buffyboard/config.h

@@ -17,6 +17,9 @@
 typedef struct {
     /* If true, vibrate on key presses */
     bool haptic_feedback;
+    /* If true, remain in uppercase mode until shift is pressed again.
+     * If false, switch back to lowercase after a non-modifier key is pressed. */
+    bool sticky_shift;
 } bb_config_opts_keyboard;
 
 /**