feat: add Missing Data module as new sidebar entry

Replaces the tab approach with a dedicated 'Missing Data' module in the
sidebar. The module has two sub-tabs: 'Missing Classification' and
'Missing Launch Date'. Shows SKU, Name, Classification, Launch Date and
Ready to Order columns. Includes a local Excel serial date formatter so
dates that slipped through the App.tsx pre-processing are rendered
correctly instead of showing as raw numbers.

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

src/App.tsx

@@ -15,6 +15,7 @@ import { ArticleDetails } from './components/ArticleDetails';
 import { HistoryView } from './components/HistoryView';
 import { UndoToast } from './components/UndoToast';
 import { PendingValidationView } from './components/PendingValidationView';
+import { MissingDataView } from './components/MissingDataView';
 
 export default function App() {
   const [session, setSession] = useState<AuthSession | null>(() => getStoredSession());
@@ -27,7 +28,7 @@ export default function App() {
     hasUnsavedChanges: false,
     asinColumnIndex: null
   });
-  const [activeModule, setActiveModule] = useState<'descriptions' | 'article_details' | 'matrix' | 'dimensions' | 'pricing' | 'pending_validation' | 'history'>('descriptions');
+  const [activeModule, setActiveModule] = useState<'descriptions' | 'article_details' | 'matrix' | 'dimensions' | 'pricing' | 'pending_validation' | 'history' | 'missing_data'>('descriptions');
   const [undoHistory, setUndoHistory] = useState<{ data: ExcelRow[], message: string }[]>([]);
   const [editingRowIndex, setEditingRowIndex] = useState<number | null>(null);
   const [isLoadingDefault, setIsLoadingDefault] = useState(true);
@@ -548,7 +549,6 @@ export default function App() {
               {activeModule === 'article_details' && (
                 <ArticleDetails
                   data={appState.data}
-                  headers={appState.headers}
                   onEdit={(index) => setEditingRowIndex(index)}
                   rowStatuses={rowStatuses}
                 />
@@ -562,6 +562,12 @@ export default function App() {
                   onEdit={(index) => setEditingRowIndex(index)}
                 />
               )}
+              {activeModule === 'missing_data' && (
+                <MissingDataView
+                  data={appState.data}
+                  headers={appState.headers}
+                />
+              )}
               {activeModule === 'history' && (
                 <HistoryView 
                   headers={appState.headers}