feat: add copy buttons to user and assistant messages

Implement hover-based copy buttons for user and assistant messages in the
Terminal component. Features include:
- Copy button appears on message hover (similar to Gemini)
- Visual "Copied!" feedback for 2 seconds after copying
- Integration with clipboard history store
- Consistent styling with existing UI components
- Excludes tool messages, system messages, and error outputs

src/lib/components/Terminal.svelte

@@ -155,6 +155,30 @@
       terminalElement.removeEventListener("copy", handleCopy);
     }
   });
+
+  // Copy message content to clipboard
+  async function copyMessage(content: string) {
+    try {
+      await navigator.clipboard.writeText(content);
+      // Optionally capture to clipboard history
+      await clipboardStore.captureClipboard(content, null, "Message from chat");
+
+      // Visual feedback could be added here if needed
+    } catch (error) {
+      console.error("Failed to copy message:", error);
+    }
+  }
+
+  // State for showing "Copied!" feedback
+  let copiedMessageId: string | null = null;
+
+  async function handleCopyMessage(messageId: string, content: string) {
+    await copyMessage(content);
+    copiedMessageId = messageId;
+    setTimeout(() => {
+      copiedMessageId = null;
+    }, 2000);
+  }
 </script>
 
 <div
@@ -185,7 +209,7 @@
       </div>
     {:else}
       {#each lines as line (line.id)}
-        <div class="terminal-line mb-2 {getLineClass(line.type)}">
+        <div class="terminal-line mb-2 {getLineClass(line.type)} relative group">
           <span class="terminal-timestamp text-xs mr-2">{formatTime(line.timestamp)}</span>
           {#if getLinePrefix(line.type)}
             <span class="terminal-prefix mr-2">{getLinePrefix(line.type)}</span>
@@ -193,11 +217,33 @@
           {#if line.toolName}
             <span class="terminal-tool-name mr-2">[{line.toolName}]</span>
           {/if}
-          {#if line.type === "assistant"}
-            <Markdown
-              content={maskPaths(line.content, hidePaths)}
-              searchQuery={currentSearchQuery}
-            />
+          {#if line.type === "assistant" || line.type === "user"}
+            <div class="message-content-wrapper">
+              <Markdown
+                content={maskPaths(line.content, hidePaths)}
+                searchQuery={currentSearchQuery}
+              />
+              <button
+                class="copy-message-btn opacity-0 group-hover:opacity-100 transition-opacity"
+                onclick={() => handleCopyMessage(line.id, line.content)}
+                title="Copy message"
+              >
+                <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">{copiedMessageId === line.id ? "Copied!" : "Copy"}</span>
+              </button>
+            </div>
           {:else}
             <HighlightedText
               content={maskPaths(line.content, hidePaths)}
@@ -267,4 +313,45 @@
     border-radius: 2px;
     padding: 0 2px;
   }
+
+  /* Message content wrapper for positioning */
+  .message-content-wrapper {
+    position: relative;
+    display: inline-block;
+    width: 100%;
+  }
+
+  /* Copy button styling */
+  .copy-message-btn {
+    position: absolute;
+    top: 0;
+    right: 0;
+    display: flex;
+    align-items: center;
+    gap: 0.4em;
+    background: var(--bg-secondary);
+    border: 1px solid var(--border-color);
+    color: var(--text-secondary);
+    padding: 0.25em 0.5em;
+    cursor: pointer;
+    border-radius: 4px;
+    font-size: 0.85em;
+    font-family: inherit;
+    transition: all 0.15s ease;
+  }
+
+  .copy-message-btn:hover {
+    background: var(--bg-hover);
+    color: var(--text-primary);
+    border-color: var(--border-hover, var(--border-color));
+  }
+
+  .copy-message-btn svg {
+    flex-shrink: 0;
+  }
+
+  /* Ensure relative positioning for parent */
+  .terminal-line {
+    position: relative;
+  }
 </style>