feat(bsr): fetch BSR.xlsx from Dropbox instead of local file

- Rewrite api/fetch-bsr.ts to proxy BSR.xlsx from the Dropbox URL,
  matching the same pattern as fetch-buybox.ts and other data sources
- Add /api/fetch-bsr proxy in vite.config.ts for local dev

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

vite.config.ts

@@ -44,6 +44,12 @@ export default defineConfig(({ mode }) => {
           changeOrigin: true,
           rewrite: () => '',
           followRedirects: true
+        },
+        '/api/fetch-bsr': {
+          target: 'https://www.dropbox.com/scl/fi/r32r9x6r7bvgg6ckiaowk/BSR.xlsx?rlkey=vka50xyhc637riq901qo5bk16&st=y6ys2oz8&dl=1',
+          changeOrigin: true,
+          rewrite: () => '',
+          followRedirects: true
         }
       }
     },