feat(webchat): add copy and edit buttons on chat messages

Copy button on all messages (clipboard API with checkmark feedback).
Edit button on user messages populates the input textarea.
Buttons appear on hover (desktop) or always visible (mobile).

src/gateway/ui/style.css

@@ -553,6 +553,15 @@ header #status.status-ok {
     min-width: 70px;
     font-size: var(--font-size-sm);
   }
+
+  .message-actions {
+    opacity: 1;
+  }
+
+  .msg-action-btn {
+    width: 28px;
+    height: 28px;
+  }
 }
 
 /* ==========================================================================
@@ -1039,6 +1048,56 @@ tr:hover td {
   border-radius: 3px;
 }
 
+/* Message action buttons (copy, edit) */
+.message {
+  position: relative;
+}
+
+.message-actions {
+  position: absolute;
+  bottom: 4px;
+  right: 4px;
+  display: flex;
+  gap: 2px;
+  opacity: 0;
+  transition: opacity var(--transition);
+}
+
+.message:hover .message-actions {
+  opacity: 1;
+}
+
+.msg-action-btn {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 24px;
+  height: 24px;
+  padding: 0;
+  border: none;
+  border-radius: 4px;
+  background: transparent;
+  color: var(--text-muted);
+  cursor: pointer;
+  transition: all var(--transition);
+}
+
+.msg-action-btn:hover {
+  background: var(--bg-tertiary);
+  color: var(--text-primary);
+}
+
+.msg-action-btn.copied {
+  color: var(--success);
+}
+
+.msg-action-btn svg {
+  width: 14px;
+  height: 14px;
+  flex-shrink: 0;
+  fill: currentColor;
+}
+
 /* Streaming text cursor */
 .streaming-cursor::after {
   content: '|';