Make Save the exercise dialog's default button

All three menu buttons were implicit submit buttons, so pressing Enter
(or the iOS keyboard's "Go") triggered implicit form submission against
the first one — the hidden Delete button — which closed the dialog and
silently discarded the typed exercise. Delete and Cancel are now
type="button", leaving Save as the form's default button.

src/index.html

@@ -277,8 +277,11 @@
           <textarea id="exercise-note" rows="5" placeholder="Reminder for how to train it, e.g. lure with a treat, mark the moment the butt touches the ground, reward"></textarea>
         </label>
         <menu>
-          <button value="delete" id="exercise-delete" class="danger" hidden>Delete</button>
+          <!-- type="button" keeps Save the form's default button, so Enter /
-          <button value="cancel" class="ghost">Cancel</button>
+               the iOS keyboard's "Go" saves instead of silently hitting the
+               (hidden) Delete button via implicit form submission. -->
+          <button type="button" value="delete" id="exercise-delete" class="danger" hidden>Delete</button>
+          <button type="button" value="cancel" class="ghost">Cancel</button>
           <button value="save" id="exercise-save">Save</button>
         </menu>
       </form>