feat: wire MKT tab to real data sources

Replace all mock data in the Profitability Dashboard with live data:
- Add 3 new API routes (fetch-deals, fetch-promos, fetch-chargebacks) proxying Dropbox Excel files
- MktDataView now accepts rawData/adsData props and fetches+parses the 3 new files on mount
- SELL OUT and units aggregated globally across all marketplaces from rawData
- PPC Spend aggregated globally from adsData; ACoS calculated as spend/sales
- Deals (col A→C), Promos (col E→K), Chargebacks (col AN→B) parsed and summed per ASIN
- Category filter populated from product line metadata
- Loading skeleton shown while files are fetching

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

App.tsx

@@ -926,7 +926,7 @@ const App: React.FC = () => {
 
               <Suspense fallback={<LoadingSpinner />}>
                 <div className={view === 'mkt' ? '' : 'hidden'}>
-                  <MktDataView />
+                  <MktDataView rawData={rawData} adsData={adsData} />
                 </div>
               </Suspense>