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/documents/DocumentPreviewModal.vue

@@ -2,6 +2,7 @@
   <!-- Overlay -->
   <div
     ref="overlayRef"
+    data-test="document-preview-modal"
     class="fixed inset-0 bg-black/60 z-50 flex flex-col"
     role="dialog"
     aria-modal="true"
@@ -9,7 +10,7 @@
     @click="handleOverlayClick"
   >
     <!-- Header bar -->
-    <div class="bg-white border-b border-gray-200 px-6 py-3 flex items-center justify-between shrink-0">
+    <div data-test="preview-modal-header" class="bg-white border-b border-gray-200 px-4 sm:px-6 py-3 flex items-center justify-between shrink-0">
       <span class="text-sm font-medium text-gray-900 truncate max-w-xs">{{ doc.filename || doc.original_name }}</span>
       <button
         @click="emit('close')"