fix(dataProcessor): parse DD/M/YY dates from column C in sell-out CSV

Add support for European day-first date format (e.g. "23/2/26" = 23 Feb 2026)
in the sell-out CSV pipeline so months and years are correctly extracted from
column C of Amazon Sell Out 2023-2025.csv.

- normalizeMonth: detect DD/MM/YY when first part > 12, return "Mon-YY"
- mapRowToRecord: add 'C', 'Date', 'Fecha', 'DATA' to month column aliases
- validateSellOutHeaders: accept date columns as substitute for YEAR + MONTH

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

vite.config.ts

@@ -1,4 +1,5 @@
 import path from 'path';
+import fs from 'fs';
 import { defineConfig, loadEnv } from 'vite';
 import react from '@vitejs/plugin-react';
 
@@ -53,7 +54,35 @@ export default defineConfig(({ mode }) => {
         }
       }
     },
-    plugins: [react()],
+    // To support fetching local 'BSR.xlsx' in Vite dev mode, we configure a custom middleware 
+    // since we use a local file instead of a dropbox link for BSR:
+    plugins: [
+      react(),
+      {
+        name: 'serve-local-bsr',
+        configureServer(server) {
+          server.middlewares.use('/api/fetch-bsr', (req, res, next) => {
+            const filePath = path.join(process.cwd(), 'BSR.xlsx');
+
+            try {
+              if (fs.existsSync(filePath)) {
+                res.setHeader('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
+                res.setHeader('Content-Disposition', 'attachment; filename="BSR.xlsx"');
+                res.setHeader('Cache-Control', 'public, max-age=3600');
+                const fileStream = fs.createReadStream(filePath);
+                fileStream.pipe(res);
+              } else {
+                res.statusCode = 404;
+                res.end(JSON.stringify({ error: 'BSR data file not found' }));
+              }
+            } catch (err) {
+              res.statusCode = 500;
+              res.end(JSON.stringify({ error: 'Failed' }));
+            }
+          });
+        }
+      }
+    ],
     define: {
       // loadEnv reads .env files; process.env has Vercel/system env vars at build time
       'process.env.API_KEY': JSON.stringify(