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/mobile.css

@@ -3333,8 +3333,10 @@ html:is([data-skin="paper-gray"], [data-skin="solarized-light"], [data-skin="cat
      recessed tray and matching pill geometry instead of reading as a fat
      accent pill parked beside a stray × glyph. Tray colors come from skin
      tokens, never a hardcoded black alpha, or the light skins get a grey slab.
-     `:has()` keeps the tray off the two sheets that carry a lone × (Session
-     Options and Add Case save from inside their own forms). */
+     `:has()` keeps the tray off Session Options, the one sheet left carrying a
+     lone × because it saves from inside its own per-section forms. Add Case
+     now has a header save of its own (its footer is hidden below 860px, so
+     that button is the only way to submit it there), and picks up the tray. */
   :is(#appSettingsModal, #sessionOptionsModal, #createCaseModal) .set-head-actions:has(.set-head-save) {
     padding: 3px;
     border: 1px solid var(--border);