fix(mobile): restore Add Case submit action

[shenlvkang-collab]

Summary

• add a compact header submit action because the <=860px layout hides the Add Case footer
• keep the desktop and compact actions synchronized across Create, Clone, Link Existing, Remote, Docker, and Manage tabs
• add a regression test and document the responsive-modal invariant

Reproduction

At 390x844, open Add Case and switch to Link Existing. In 1.24.0 the shared compact CSS hides .set-foot, which contains the only Link button, while Add Case has no header replacement. Selecting a directory therefore returns to a form that cannot be submitted.

Verification

• npm test — 325 files passed, 1 skipped; 6,341 tests passed, 12 skipped
• npm run typecheck
• npm run lint
• npm run check:frontend-syntax
• npm run check:public-assets
• npm run format:check
• npm run check:lockfile
• npm run build
• Playwright at 390x844: Browse -> Select Current Folder -> Select -> Link successfully linked an isolated existing directory