Add digital e-signature waiver + update How It Works to 5 steps

- waiver.php: full rental agreement with canvas e-signature pad, 6 required
  checkboxes, typed name field; stores sig image + IP + timestamp in DB;
  emails signed confirmation to customer and admin
- bookings table: add waiver_signed, waiver_signed_at, waiver_ip, waiver_name,
  waiver_sig columns
- contact.php: confirmation email now includes Sign Rental Agreement button/link
- admin/index.php: Waiver column shows Signed (date) or Pending + Send Link
- index.html: How It Works expanded to 5 steps (added Get Approved + Sign Waiver
  before Hit the Road); insurance updated to Proof of insurance required; FAQ
  and JSON-LD updated to match

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

contact.php

@@ -100,6 +100,11 @@ $confirmHtml = "<div style='max-width:600px;margin:0 auto;font-family:Arial,sans
       <p style='margin:4px 0;font-size:14px;color:#374151'><strong>Requested Date:</strong> {$dateLabel}</p>
       <p style='margin:4px 0;font-size:14px;color:#374151'><strong>Total:</strong> {$amountLabel}</p>
     </div>
+    <div style='margin:20px 0;padding:16px;background:#fff7ed;border:1px solid #fed7aa;border-radius:10px;text-align:center'>
+      <p style='margin:0 0 10px;font-size:14px;font-weight:700;color:#111'>Next Step: Sign Your Rental Agreement</p>
+      <p style='margin:0 0 14px;font-size:13px;color:#6b7280'>Once your booking is confirmed you'll sign our digital waiver online — no printer needed. Your link:</p>
+      <a href='https://parkerslingshotrentals.com/waiver.php?ref={$ref}' style='display:inline-block;background:#f97316;color:#fff;text-decoration:none;padding:10px 24px;border-radius:6px;font-weight:700;font-size:14px'>Sign Rental Agreement &rarr;</a>
+    </div>
     <p style='color:#374151'>Questions? Call or text <strong>(817) 555-0199</strong> or reply to this email.</p>
     <p style='color:#374151'>Ride on,<br><strong>The Parker County Slingshot Team</strong></p>
   </div>