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/js/utils.js

@@ -650,4 +650,34 @@ function initEditor() {
   );
 }
 
-export { getFileIcon, safeCreateIcons, flushIcons, safeHighlight, escapeHtml, EXT_ICONS, openEditor, closeEditor, saveFile, deleteFile, waitForCodeMirror, initEditor };
+function safeHighlight(block) {
+  try { hljs.highlightElement(block); } catch (e) { /* skip */ }
+}
+
+/** Copy text to clipboard — works on both HTTP and HTTPS */
+function copyToClipboard(text) {
+  // Try modern API first (HTTPS-only)
+  if (navigator.clipboard && window.isSecureContext) {
+    navigator.clipboard.writeText(text).catch(() => {});
+    return true;
+  }
+  // Fallback for HTTP contexts
+  const ta = document.createElement("textarea");
+  ta.value = text;
+  ta.style.position = "fixed";
+  ta.style.left = "-9999px";
+  ta.style.top = "-9999px";
+  document.body.appendChild(ta);
+  ta.focus();
+  ta.select();
+  try {
+    document.execCommand("copy");
+    return true;
+  } catch (e) {
+    return false;
+  } finally {
+    document.body.removeChild(ta);
+  }
+}
+
+export { getFileIcon, safeCreateIcons, flushIcons, safeHighlight, escapeHtml, EXT_ICONS, openEditor, closeEditor, saveFile, deleteFile, waitForCodeMirror, initEditor, copyToClipboard };