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>

api/fetch-promos.ts

@@ -0,0 +1,37 @@
+import type { VercelRequest, VercelResponse } from '@vercel/node';
+
+const PROMOS_DROPBOX_URL = "https://www.dropbox.com/scl/fi/ufhej9do9w839oyyfrpk3/Promos_Amazon_2025_all-countries.xlsx?rlkey=4foano9dt5h3sl58l35vqg89d&st=wu0t56wh&dl=1";
+
+export default async function handler(req: VercelRequest, res: VercelResponse) {
+    res.setHeader('Access-Control-Allow-Origin', '*');
+    res.setHeader('Access-Control-Allow-Methods', 'GET, OPTIONS');
+    res.setHeader('Access-Control-Allow-Headers', 'Content-Type');
+
+    if (req.method === 'OPTIONS') {
+        return res.status(200).end();
+    }
+
+    try {
+        console.log('[fetch-promos] Fetching Promos from Dropbox...');
+        const response = await fetch(PROMOS_DROPBOX_URL, {
+            cache: 'no-store',
+            headers: {
+                'Pragma': 'no-cache',
+                'Cache-Control': 'no-cache'
+            }
+        });
+
+        if (!response.ok) {
+            throw new Error(`Dropbox responded with ${response.status}`);
+        }
+
+        const buffer = await response.arrayBuffer();
+        console.log('[fetch-promos] Successfully fetched Promos Excel, size:', buffer.byteLength);
+
+        res.setHeader('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
+        res.status(200).send(Buffer.from(buffer));
+    } catch (error: any) {
+        console.error('[fetch-promos] Error:', error);
+        res.status(500).json({ error: error.message });
+    }
+}