fix: add clipboard fallback for code block copy in web UI

The Clipboard API (navigator.clipboard.writeText) requires a
secure context (HTTPS or localhost). When the web UI is accessed
over plain HTTP or on platforms without clipboard support (e.g.,
Termux), the copy button silently fails.

Add a fallback using document.execCommand('copy') with a
temporary textarea element for non-secure contexts.

Fixes #1340

Author: Br1an67

web/src/components/ai-elements/code-block.tsx

@@ -508,13 +508,31 @@ export const CodeBlockCopyButton = ({
   const { code } = useContext(CodeBlockContext);
 
   const copyToClipboard = async () => {
-    if (typeof window === "undefined" || !navigator?.clipboard?.writeText) {
+    if (typeof window === "undefined") {
       onError?.(new Error("Clipboard API not available"));
       return;
     }
 
     try {
-      await navigator.clipboard.writeText(code);
+      if (navigator?.clipboard?.writeText) {
+        await navigator.clipboard.writeText(code);
+      } else {
+        // Fallback for non-secure contexts (HTTP) or older browsers
+        const textarea = document.createElement("textarea");
+        textarea.value = code;
+        textarea.style.position = "fixed";
+        textarea.style.opacity = "0";
+        document.body.appendChild(textarea);
+        try {
+          textarea.select();
+          const success = document.execCommand("copy");
+          if (!success) {
+            throw new Error("execCommand copy failed");
+          }
+        } finally {
+          document.body.removeChild(textarea);
+        }
+      }
       setIsCopied(true);
       onCopy?.();
       setTimeout(() => setIsCopied(false), timeout);