fix: HTTP fallback for clipboard — execCommand('copy') textarea method

- Added copyToClipboard() in utils.js with dual strategy:
  - HTTPS: navigator.clipboard.writeText() (modern API)
  - HTTP: textarea + execCommand('copy') (legacy fallback)
- viewer.js: uses copyToClipboard() instead of direct navigator.clipboard
- popout.html: same dual-strategy inline
- Fixed: 'Erreur lors de la copie' on HTTP contexts

frontend/popout.html

@@ -913,11 +913,26 @@ const RightSidebarManager = {
             btn.addEventListener('click', function(e) {
               e.stopPropagation();
               var code = (pre.querySelector('code') || pre).textContent;
-              navigator.clipboard.writeText(code).then(function() {
+              var ok = false;
+              // Try modern API first (HTTPS)
+              if (navigator.clipboard && window.isSecureContext) {
+                navigator.clipboard.writeText(code).then(function() { ok = true; }).catch(function() {});
+              } else {
+                // Fallback for HTTP
+                var ta = document.createElement('textarea');
+                ta.value = code;
+                ta.style.position = 'fixed';
+                ta.style.left = '-9999px';
+                document.body.appendChild(ta);
+                ta.focus(); ta.select();
+                try { document.execCommand('copy'); ok = true; } catch(er) {}
+                document.body.removeChild(ta);
+              }
+              if (ok) {
                 btn.textContent = '✓ Copié';
                 btn.classList.add('copied');
                 setTimeout(function() { btn.textContent = 'Copier'; btn.classList.remove('copied'); }, 2000);
-              }).catch(function() {});
+              }
             });
             wrapper.appendChild(btn);
           });