Set maximum digit length to 9

The type of calc->stack[0] is int,
and the maximum value for an int is 2,147,483,647.
Conservatively, we ensure that when the length reaches 9 digits,
pressing the digit button will not increase the number of digits.

apps/calc.c

@@ -119,8 +119,11 @@ static void _apps_calc_digit(apps_calc_t *calc, int digit)
         calc->stack[0] = 0;
         calc->pending_delete = false;
     }
-    calc->stack[0] = calc->stack[0] * 10 + digit;
-    _apps_calc_update_value(calc);
+
+    if (calc->stack[0] < 100000000) {
+        calc->stack[0] = calc->stack[0] * 10 + digit;
+        _apps_calc_update_value(calc);
+    }
 }
 
 static void _apps_calc_button_signal(twin_button_t *button,