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

[shenlvkang-collab]

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

The bug

On a phone, the Add Case modal cannot be submitted at all — the Create/Link button does not
exist anywhere on screen.

mobile.css hides #createCaseModal's .set-foot below 860px:

:is(#appSettingsModal, #sessionOptionsModal, #createCaseModal) .set-foot { display: none; }

and, unlike the Settings modal, that modal's header carries no set-head-save. So the only
submit control is the one that was just hidden. Filling in a case name and tapping around
does nothing; the sheet can only be dismissed.

The fix

Add the header button, following the Settings modal's existing pattern (close first in the
DOM so the focus trap still lands on it; row-reverse puts save to its left), and drive both
buttons together in switchCaseModalTab() and submitCaseModal() — so whichever one is
pressed, the other shows the same pending label and is equally disabled. That matters because
the clone path holds the request open for minutes and a second click starts a second clone.

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 one CSS edit is a comment: mobile.css still described Add Case as one of "the two sheets
that carry a lone ×", which this change makes false, and it is exactly the comment a reviewer
would check.

Verification

Isolated worktree, clean npm ci:

npm run typecheck              clean
npm run format:check           All matched files use Prettier code style!
npm run check:frontend-syntax  ✓ 34 frontend JS files parse cleanly
npm run check:public-assets    Public asset checks passed (42 files).
npm test                       Test Files 325 passed | 1 skipped (326)
                               Tests 6340 passed | 12 skipped (6352)

Found while fixing the response viewer (#365) on a phone; unrelated to it, so it is its own PR.