fix(mobile): give the Add Case modal a reachable submit button

mobile.css hides #createCaseModal's .set-foot below 860px, and that modal's
header — unlike Settings' — carries no set-head-save. So on a phone the
Create/Link button existed nowhere and the modal could not be submitted at all.

Adds the header button and drives both together through switchCaseModalTab()
and submitCaseModal(), so whichever one is pressed the other shows the same
pending state and is equally unclickable. Following the Settings pattern also
means Add Case picks up the existing .set-head-actions:has(.set-head-save) tray
and .set-head-save sizing with no new CSS; the mobile.css comment that still
listed Add Case as a lone-× sheet is corrected to match.

src/web/public/index.html

@@ -2582,8 +2582,15 @@
       <div class="modal-content modal-lg set-shell">
         <div class="modal-header set-shell-head">
           <h3>Add Case</h3>
+          <!-- mobile.css hides this modal's .set-foot, so on a phone the footer's
+               Create/Link button is unreachable and the modal cannot be submitted
+               at all. Mirrors the Settings modal's header Save: close first in the
+               DOM so the focus trap still lands on it, row-reverse puts this to
+               its left. Both buttons are driven together by switchCaseModalTab()
+               and submitCaseModal(). -->
           <div class="set-head-actions">
             <button class="modal-close" onclick="app.closeCreateCaseModal()" aria-label="Close create case">&times;</button>
+            <button class="set-head-save" id="caseModalSubmitMobile" onclick="app.submitCaseModal()">Create</button>
           </div>
         </div>
         <div class="set-body">