security: fix all Medium findings from FLAWS_REPORT audit

- Auth: TOTP replay protection with counter tracking, constant-time
  backup code comparison, atomic lockout increment, per-token logout
- Invoices/PDFs: net-based VAT calculation, dangerous URL scheme
  stripping in cleanQuillHtml, orders-pdf error handling
- Orders: reject item changes on status transition, cascading
  delete cleanup, take:1 with orderBy
- Projects: atomic rename collision handling, MIME/extension
  validation, empty customer name rejection
- Attendance: Czech public holiday awareness in frontend fund
  calculation, leave_hours 0 handling, invalid date NaN guard,
  bounded per-month queries in workfund
- Users/Admin: profile audit logging + password validation, session
  revocation guard, session ID validation, dashboard DB aggregation,
  soft-deleted record protection in scope templates
- Frontend: FormField label linkage, Pagination ARIA, error
  handling in OrderConfirmationModal, 401 propagation, GPS emoji
  hidden from screen readers, table sort state fix, geolocation
  race/abort cleanup, Leaflet popup DOM safety, Vehicles toggleActive
  minimal body, CompanySettings ref mutation fix, OfferDetail unlock
  abort, AttendanceBalances combined fetches
- Utils: env validation, Puppeteer concurrency mutex, invoice alert
  cron cleanup on shutdown, body limit alignment, TOTP error logging,
  trustProxy from env, symlink rejection, rate cache Map usage

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/utils/html-to-pdf.ts

@@ -1,31 +1,41 @@
 import { Browser } from "puppeteer";
 
 let browser: Browser | null = null;
+let launching: Promise<Browser> | null = null;
 
 async function getBrowser(): Promise<Browser> {
   if (browser && browser.connected) return browser;
 
-  // Try puppeteer (bundles Chromium), fall back to puppeteer-core (system Chromium)
-  try {
-    const puppeteer = await import("puppeteer");
-    browser = await puppeteer.default.launch({
-      headless: true,
-      args: ["--no-sandbox", "--disable-setuid-sandbox", "--disable-gpu"],
-    });
-  } catch {
-    const core = await import("puppeteer-core");
-    const executablePath =
-      process.env.CHROMIUM_PATH ||
-      "/usr/bin/chromium-browser" ||
-      "/usr/bin/chromium";
-    browser = await core.default.launch({
-      headless: true,
-      executablePath,
-      args: ["--no-sandbox", "--disable-setuid-sandbox", "--disable-gpu"],
-    });
-  }
+  if (launching) return launching;
 
-  return browser;
+  launching = (async () => {
+    // Try puppeteer (bundles Chromium), fall back to puppeteer-core (system Chromium)
+    try {
+      const puppeteer = await import("puppeteer");
+      browser = await puppeteer.default.launch({
+        headless: true,
+        args: ["--no-sandbox", "--disable-setuid-sandbox", "--disable-gpu"],
+      });
+    } catch {
+      const core = await import("puppeteer-core");
+      const executablePath =
+        process.env.CHROMIUM_PATH ||
+        "/usr/bin/chromium-browser" ||
+        "/usr/bin/chromium";
+      browser = await core.default.launch({
+        headless: true,
+        executablePath,
+        args: ["--no-sandbox", "--disable-setuid-sandbox", "--disable-gpu"],
+      });
+    }
+    return browser!;
+  })();
+
+  try {
+    return await launching;
+  } finally {
+    launching = null;
+  }
 }
 
 export async function htmlToPdf(html: string): Promise<Buffer> {