feat: Add UK inventory sync from Dropbox

- Create new API endpoint api/fetch-uk-inventory.ts
- Add processUKInventoryExcel function in dataProcessor.ts
- Update handleVendorStockFetch to load PANEU + UK in parallel
- Configure Vite proxy for UK inventory endpoint
- UK stock correctly populates 'uk' field in vendorStockMap

api/fetch-uk-inventory.ts

@@ -0,0 +1,39 @@
+import type { VercelRequest, VercelResponse } from '@vercel/node';
+
+// UK Inventory from Dropbox (using dl=1 for direct download)
+const UK_INVENTORY_DROPBOX_URL = "https://www.dropbox.com/scl/fi/an1ldjmtej7tbt6fuh2ut/UK-Inventory.xlsx?rlkey=5h8e4st885bggibujk0b1lmm8&st=7vbagnwz&dl=1";
+
+export default async function handler(req: VercelRequest, res: VercelResponse) {
+    // CORS headers
+    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-uk-inventory] Fetching UK Inventory from Dropbox...');
+        const response = await fetch(UK_INVENTORY_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-uk-inventory] Successfully fetched UK Inventory 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-uk-inventory] Error:', error);
+        res.status(500).json({ error: error.message });
+    }
+}