visually indicate when "qnh from plane" button is available

html/index.html

@@ -910,7 +910,7 @@
                           <span> <input id="altimeter_input" type="number" step="0.01" value="29.92" class="searchInput"></span>
                           <span> <button class="formButton" type="submit">Set</button></span>
                           <span> <button class="formButton" type="button" id="altimeter_set_standard">Standard</button>
-                          <span> <button class="formButton" type="button" id="altimeter_set_selected">From Plane</button>
+                          <span> <button class="formButton" type="button" disabled id="altimeter_set_selected">From Plane</button>
                         </form>
                       </td>
                     </tr>

html/script.js

@@ -3368,6 +3368,12 @@ let somethingSelected = false;
 function refreshSelected() {
     const selected = SelectedPlane;
 
+    if (!selected || !selected.nav_qnh) {
+        jQuery('#altimeter_set_selected').prop("disabled", true);
+    } else {
+        jQuery('#altimeter_set_selected').prop("disabled", false);
+    }
+
     if (!selected) {
         if (somethingSelected) {
             adjustInfoBlock();

html/style.css

@@ -350,10 +350,13 @@ select {
 
 .formButton {
     font-size: var(--FS2);
-    color:var(--TXTCOLOR3);
+    color:var(--TXTCOLOR2);
     background-color: var(--BGCOLOR2);
     border-color: var(--BGCOLOR1);
 }
+.formButton:disabled {
+    color:var(--TXTCOLOR3);
+}
 
 select.error, textarea.error, input.error {
     color: #FF0000;