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

vite.config.ts

@@ -20,6 +20,12 @@ export default defineConfig(({ mode }) => {
           changeOrigin: true,
           rewrite: () => '',
           followRedirects: true
+        },
+        '/api/fetch-uk-inventory': {
+          target: 'https://www.dropbox.com/scl/fi/an1ldjmtej7tbt6fuh2ut/UK-Inventory.xlsx?rlkey=5h8e4st885bggibujk0b1lmm8&st=7vbagnwz&dl=1',
+          changeOrigin: true,
+          rewrite: () => '',
+          followRedirects: true
         }
       }
     },