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/styles.css

@@ -2213,6 +2213,10 @@ body.bg-bubbles {
   display: flex; flex-direction: column;
   background: #fff;
 }
+body.docs-editor-open .kid-hero,
+body.docs-editor-open .kid-nav,
+body.docs-editor-open .page-confetti,
+body.docs-editor-open .tab-panel { display: none !important; }
 #docs-editor-frame { flex: 1; min-height: 0; }
 #docs-editor-frame iframe { width: 100%; height: 100%; border: none; display: block; }
 
@@ -2243,3 +2247,11 @@ body.bg-bubbles {
 
 .docs-error { padding: 1.5rem; color: var(--muted); font-size: .9rem; }
 .docs-error code { background: var(--surface); padding: .1rem .4rem; border-radius: 4px; }
+
+/* Admin doc viewer */
+#admin-doc-viewer {
+  position: fixed; inset: 0; z-index: 10000;
+  display: flex; flex-direction: column; background: #fff;
+}
+#admin-doc-frame { flex: 1; min-height: 0; }
+#admin-doc-frame iframe { width: 100%; height: 100%; border: none; display: block; }