fix: handle clipboard errors and cleanup timeout in CopyButton

thoughts-frontend/components/copy-button.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { Check, Copy } from "lucide-react";
 import { cn } from "@/lib/utils";
 
@@ -11,17 +11,30 @@ interface CopyButtonProps {
 
 export function CopyButton({ text, className }: CopyButtonProps) {
   const [copied, setCopied] = useState(false);
+  const timeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  useEffect(() => {
+    return () => {
+      if (timeoutRef.current) clearTimeout(timeoutRef.current);
+    };
+  }, []);
 
   const handleCopy = async () => {
-    await navigator.clipboard.writeText(text);
-    setCopied(true);
-    setTimeout(() => setCopied(false), 1500);
+    if (copied) return;
+    try {
+      await navigator.clipboard.writeText(text);
+      setCopied(true);
+      timeoutRef.current = setTimeout(() => setCopied(false), 1500);
+    } catch {
+      // clipboard unavailable — silently no-op
+    }
   };
 
   return (
     <button
       onClick={handleCopy}
       title="Copy to clipboard"
+      aria-label="Copy to clipboard"
       className={cn(
         "inline-flex items-center justify-center rounded p-0.5 text-muted-foreground/60 transition-colors hover:text-muted-foreground",
         className