fix: QA audit bugs BUG-051/052/056/057/059/062/063/064/065/066/068

- BUG-056: Fix sitemap namespace sitemapns.org -> sitemaps.org
- BUG-062: Extend <main> to wrap all page content (hero+features+pricing+EU section)
- BUG-064: Add sr-only <label> elements to all modal form inputs (signup, recovery, change-email)
- BUG-051/052: Remove duplicate X-Content-Type-Options headers from nginx (let helmet handle)
- BUG-057: Fix JSON-LD and pricing card: Pro plan is 2,500 PDFs/month not 5,000
- BUG-059: Add meta description, canonical URL, og: tags to /docs page
- BUG-063: Change eu-hosting h3 to h2 (correct heading hierarchy)
- BUG-065/066: Add aria-modal=true, role=dialog to modals; aria-label=Close to close buttons
- BUG-068: Add hash-based modal open for #change-email on page load
- Add .sr-only CSS utility class to base and index styles

public/partials/_modals.html

@@ -1,12 +1,13 @@
 <!-- Signup Modal -->
 <div class="modal-overlay" id="signupModal" role="dialog" aria-label="Sign up for API key">
   <div class="modal">
-    <button class="close" id="btn-close-signup">&times;</button>
+    <button class="close" id="btn-close-signup" aria-label="Close">&times;</button>
 
     <div id="signupInitial" class="active">
       <h2>Get your free API key</h2>
       <p>Enter your email to get started. No credit card required.</p>
       <div class="signup-error" id="signupError"></div>
+      <label for="signupEmail" class="sr-only">Email address</label>
       <input type="email" id="signupEmail" placeholder="your.email@example.com" style="width:100%;padding:12px;border:1px solid var(--border);background:var(--bg);color:var(--fg);border-radius:8px;font-size:0.9rem;margin-bottom:16px;" required>
       <button class="btn btn-primary" style="width:100%" id="signupBtn">Generate API Key →</button>
       <p style="margin-top:16px;color:var(--muted);font-size:0.8rem;text-align:center;">100 free PDFs/month • All endpoints included<br><a href="#" class="open-recover" style="color:var(--muted)">Lost your API key? Recover it →</a></p>
@@ -21,6 +22,7 @@
       <h2>Enter verification code</h2>
       <p>We sent a 6-digit code to <strong id="verifyEmailDisplay"></strong></p>
       <div class="signup-error" id="verifyError"></div>
+      <label for="verifyCode" class="sr-only">Verification code</label>
       <input type="text" id="verifyCode" placeholder="123456" maxlength="6" pattern="[0-9]{6}" inputmode="numeric" style="width:100%;padding:14px;border:1px solid var(--border);background:var(--bg);color:var(--fg);border-radius:8px;font-size:1.4rem;letter-spacing:0.3em;text-align:center;margin-bottom:16px;font-family:monospace;" required>
       <button class="btn btn-primary" style="width:100%" id="verifyBtn">Verify →</button>
       <p style="margin-top:16px;color:var(--muted);font-size:0.8rem;text-align:center;">Code expires in 15 minutes</p>