Fix editor full-screen and add parent-side doc viewer

Student editor:
- Add body.docs-editor-open class when editor opens; hide hero, nav,
  confetti so nothing bleeds behind the fixed overlay

Admin docs panel:
- New "Documents" sidebar entry listing all submitted docs with student
  name, date, and unread badge
- Click any doc to open it in OnlyOffice in view (read-only) mode
- Download button for offline access
- /api/docs/bykey/:key/editor-config server route for admin viewer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/admin.html

@@ -57,6 +57,9 @@
         <button class="nav-pill" data-panel="decks">
           <span class="nav-icon">🗂️</span><span class="nav-label">Flashcards</span>
         </button>
+        <button class="nav-pill" data-panel="docs">
+          <span class="nav-icon">📝</span><span class="nav-label">Docs</span>
+        </button>
       </nav>
       <div class="admin-sidebar-footer">
         <button class="nav-pill" data-panel="settings">
@@ -679,6 +682,23 @@
         </div>
       </section>
 
+      <!-- Docs -->
+      <section id="panel-docs" class="panel" hidden>
+        <h1>Submitted Documents</h1>
+        <div id="admin-docs-list">
+          <p class="hint">Loading…</p>
+        </div>
+        <!-- Full-screen doc viewer -->
+        <div id="admin-doc-viewer" hidden>
+          <div class="docs-editor-toolbar">
+            <button id="btn-admin-doc-back" class="docs-back-btn">← Back to list</button>
+            <span id="admin-doc-viewer-title" class="docs-editor-title-text"></span>
+            <a id="admin-doc-download" class="docs-submit-btn" download>⬇ Download</a>
+          </div>
+          <div id="admin-doc-frame"></div>
+        </div>
+      </section>
+
       <!-- Settings -->
       <section id="panel-settings" class="panel">
         <h1>Settings</h1>