feat: add copy button to code blocks

Adds a convenient copy-to-clipboard button on all code snippets in
markdown output. Each code block now displays:
- Language label in the header
- Copy button with visual feedback ("Copied!" for 2 seconds)

Refs #68

src/lib/components/Markdown.svelte

@@ -17,7 +17,20 @@
   renderer.code = ({ text, lang }) => {
     const language = lang && hljs.getLanguage(lang) ? lang : "plaintext";
     const highlighted = hljs.highlight(text, { language }).value;
-    return `<pre class="hljs-code-block"><code class="hljs language-${language}">${highlighted}</code></pre>`;
+    const escapedText = text.replace(/"/g, "&quot;").replace(/</g, "&lt;").replace(/>/g, "&gt;");
+    return `<div class="code-block-wrapper">
+      <div class="code-block-header">
+        <span class="code-block-lang">${language}</span>
+        <button class="copy-code-btn" data-code="${escapedText}" title="Copy code">
+          <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+            <rect x="9" y="9" width="13" height="13" rx="2" ry="2"></rect>
+            <path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1"></path>
+          </svg>
+          <span class="copy-text">Copy</span>
+        </button>
+      </div>
+      <pre class="hljs-code-block"><code class="hljs language-${language}">${highlighted}</code></pre>
+    </div>`;
   };
 
   renderer.codespan = ({ text }) => {
@@ -123,6 +136,28 @@
     }
   }
 
+  async function handleCopyClick(event: MouseEvent) {
+    const target = event.target as HTMLElement;
+    const copyBtn = target.closest(".copy-code-btn") as HTMLButtonElement;
+    if (copyBtn) {
+      event.preventDefault();
+      const code = copyBtn.dataset.code
+        ?.replace(/&quot;/g, '"')
+        .replace(/&lt;/g, "<")
+        .replace(/&gt;/g, ">");
+      if (code) {
+        await navigator.clipboard.writeText(code);
+        const textSpan = copyBtn.querySelector(".copy-text");
+        if (textSpan) {
+          textSpan.textContent = "Copied!";
+          setTimeout(() => {
+            textSpan.textContent = "Copy";
+          }, 2000);
+        }
+      }
+    }
+  }
+
   onMount(() => {
     if (containerElement) {
       containerElement.querySelectorAll("pre code:not(.hljs)").forEach((block) => {
@@ -138,6 +173,7 @@
   onclick={(e) => {
     handleSpoilerClick(e);
     handleLinkClick(e);
+    handleCopyClick(e);
   }}
   onkeydown={handleSpoilerKeydown}
   role="presentation"
@@ -163,13 +199,59 @@
     margin-bottom: 0;
   }
 
+  .markdown-content :global(.code-block-wrapper) {
+    margin: 0.75em 0;
+    border-radius: 6px;
+    border: 1px solid var(--border-color);
+    overflow: hidden;
+  }
+
+  .markdown-content :global(.code-block-header) {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    background: var(--bg-secondary);
+    padding: 0.4em 0.75em;
+    border-bottom: 1px solid var(--border-color);
+    font-size: 0.8em;
+  }
+
+  .markdown-content :global(.code-block-lang) {
+    color: var(--text-secondary);
+    font-family: "JetBrains Mono", "Fira Code", monospace;
+    text-transform: lowercase;
+  }
+
+  .markdown-content :global(.copy-code-btn) {
+    display: flex;
+    align-items: center;
+    gap: 0.4em;
+    background: transparent;
+    border: none;
+    color: var(--text-secondary);
+    cursor: pointer;
+    padding: 0.25em 0.5em;
+    border-radius: 4px;
+    font-size: 0.9em;
+    transition: all 0.15s ease;
+  }
+
+  .markdown-content :global(.copy-code-btn:hover) {
+    background: var(--bg-hover);
+    color: var(--text-primary);
+  }
+
+  .markdown-content :global(.copy-code-btn svg) {
+    flex-shrink: 0;
+  }
+
   .markdown-content :global(.hljs-code-block) {
     background: var(--bg-code, #1e1e2e);
-    border-radius: 6px;
+    border-radius: 0;
     padding: 1em;
-    margin: 0.75em 0;
+    margin: 0;
     overflow-x: auto;
-    border: 1px solid var(--border-color);
+    border: none;
   }
 
   .markdown-content :global(.hljs-code-block code) {