fix: auto-detect system Chromium for puppeteer on Linux servers

Puppeteer's bundled Chrome often fails on Linux servers due to
architecture mismatches. Now auto-detects system-installed Chromium
at common paths (/usr/bin/chromium-browser, /usr/bin/chromium), or
honors PUPPETEER_EXECUTABLE_PATH env var.

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

scripts/fetch-cd-rates.ts

@@ -402,9 +402,20 @@ async function main() {
   let browser: Browser | null = null;
 
   try {
+    // Use system Chromium if PUPPETEER_EXECUTABLE_PATH is set,
+    // or auto-detect common locations on Linux servers.
+    const executablePath =
+      process.env.PUPPETEER_EXECUTABLE_PATH ||
+      ['/usr/bin/chromium-browser', '/usr/bin/chromium', '/usr/bin/google-chrome'].find(
+        (p) => { try { require('fs').accessSync(p); return true; } catch { return false; } },
+      ) ||
+      undefined;
+
     console.log('\nLaunching headless browser...');
+    if (executablePath) console.log(`Using browser: ${executablePath}`);
     browser = await puppeteer.launch({
       headless: true,
+      executablePath,
       args: [
         '--no-sandbox',
         '--disable-setuid-sandbox',