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/components/cloud/CloudCredentialModal.vue

@@ -5,7 +5,7 @@
     @click.self="handleOverlayClick"
     @keydown.escape.window="handleEscape"
   >
-    <div class="bg-white rounded-xl shadow-xl w-full max-w-md p-6">
+    <div data-test="cloud-credential-modal-panel" class="bg-white rounded-xl shadow-xl w-full max-w-md p-6 max-h-[90vh] overflow-y-auto">
       <!-- Header -->
       <div class="flex items-center justify-between mb-5">
         <h3 class="text-xl font-semibold text-gray-900">