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/viewer.js

@@ -681,16 +681,16 @@ export function renderFile(data) {
     wrapper.appendChild(pre);
 
     const btn = el("button", { class: "copy-code-btn", title: "Copier le code" }, [document.createTextNode("Copier")]);
-    btn.addEventListener("click", async (e) => {
+    btn.addEventListener("click", (e) => {
       e.stopPropagation();
       const code = pre.querySelector("code")?.textContent || pre.textContent;
-      try {
-        await navigator.clipboard.writeText(code);
+      const copied = copyToClipboard(code);
+      if (copied) {
         btn.textContent = "✓ Copié";
         btn.classList.add("copied");
         if (typeof showToast === "function") showToast("Code copié dans le presse-papiers", "success");
         setTimeout(() => { btn.textContent = "Copier"; btn.classList.remove("copied"); }, 2000);
-      } catch {
+      } else {
         if (typeof showToast === "function") showToast("Erreur lors de la copie", "error");
       }
     });