feat: auto-load Traffic data from Dropbox on app startup

- api/fetch-traffic.ts: New endpoint to fetch Traffic Weekly.xlsx from Dropbox
- App.tsx: Added handleTrafficFetch function and auto-fetch on initialization

api/fetch-traffic.ts

@@ -0,0 +1,32 @@
+import type { VercelRequest, VercelResponse } from '@vercel/node';
+
+const TRAFFIC_DROPBOX_URL = "https://www.dropbox.com/scl/fi/9cenju8djxav9z6q5if6m/Traffic-Weekly.xlsx?rlkey=r2oaixmnvzq6nnjuus0uowa6l&st=r7knws3r&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-traffic] Fetching Traffic from Dropbox...');
+        const response = await fetch(TRAFFIC_DROPBOX_URL);
+
+        if (!response.ok) {
+            throw new Error(`Dropbox responded with ${response.status}`);
+        }
+
+        const buffer = await response.arrayBuffer();
+        console.log('[fetch-traffic] Successfully fetched Traffic 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-traffic] Error:', error);
+        res.status(500).json({ error: error.message });
+    }
+}