feat(11-04): mobile-safe modals and form baseline verification

- Add max-h-[90vh] overflow-y-auto to ShareModal, CloudCredentialModal,
  FolderDeleteModal, and DocumentView cloud-delete modal panels
- Add responsive px-4 sm:px-6 to DocumentPreviewModal header for narrow viewports
- Add data-test attributes to all modal panels for testability
- Confirm @tailwindcss/forms active in tailwind.config.js (no drift found)
- Audit forms: focus:outline-none focus:ring-2 pattern is consistent throughout
- Add 4 test files covering mobile-safe modal classes and form baseline:
  ShareModal.mobile.test.js, CloudCredentialModal.mobile.test.js,
  FolderDeleteModal.mobile.test.js, DocumentPreviewModal.mobile.test.js

VISUAL-02, RESP-04

frontend/src/views/DocumentView.vue

@@ -119,7 +119,8 @@
         role="dialog"
         aria-modal="true"
         aria-labelledby="cloud-delete-modal-title"
-        class="bg-white rounded-2xl shadow-xl p-6 max-w-sm w-full mx-4"
+        data-test="cloud-delete-modal-panel"
+        class="bg-white rounded-2xl shadow-xl p-6 max-w-sm w-full mx-4 max-h-[90vh] overflow-y-auto"
       >
         <div class="flex items-center gap-2 mb-2">
           <AppIcon name="warning" class="w-5 h-5 text-amber-500 shrink-0" />