chore: switch branch indicator to main and auto-sync Vercel build branch

scripts/build-vercel.cjs

@@ -5,6 +5,22 @@ const path = require('node:path');
 const root = path.resolve(__dirname, '..');
 const output = path.join(root, 'dist');
 fs.mkdirSync(output, { recursive: true });
+
+// If building on Vercel, update version.json with the deployed branch
+const vercelBranch = process.env.VERCEL_GIT_COMMIT_REF;
+const versionFile = path.join(root, 'version.json');
+if (vercelBranch && fs.existsSync(versionFile)) {
+  try {
+    const data = JSON.parse(fs.readFileSync(versionFile, 'utf8'));
+    data.branch = vercelBranch;
+    data.updatedAt = new Date().toISOString();
+    fs.writeFileSync(versionFile, JSON.stringify(data, null, 2) + '\n');
+    console.log(`[build-vercel] Synced branch to: ${vercelBranch}`);
+  } catch (err) {
+    console.warn('[build-vercel] Could not update branch:', err.message);
+  }
+}
+
 for (const name of ['index.html', 'version.json', 'assets', 'css', 'js']) {
   const source = path.join(root, name);
   if (!fs.existsSync(source)) throw new Error(`Missing static application input: ${name}`);