feat: render chat messages as markdown with code blocks

- Add react-markdown and remark-gfm for markdown rendering
- Create MarkdownContent component with custom code block renderer
  featuring language labels and copy-to-clipboard buttons
- Add full markdown prose styles (headings, lists, tables, links,
  blockquotes, inline code) matching the dark zinc theme
- Replace plain-text message rendering in ChatPane with MarkdownContent
- Handle fenced code blocks with and without language specifiers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/styles.css

@@ -102,3 +102,108 @@ textarea {
   min-height: 44px;
   max-height: 200px;
 }
+
+/* Markdown prose styles for chat messages */
+.markdown-content {
+  line-height: 1.7;
+  word-break: break-word;
+}
+
+.markdown-content > *:first-child {
+  margin-top: 0;
+}
+
+.markdown-content > *:last-child {
+  margin-bottom: 0;
+}
+
+.markdown-content h1,
+.markdown-content h2,
+.markdown-content h3,
+.markdown-content h4,
+.markdown-content h5,
+.markdown-content h6 {
+  font-weight: 600;
+  color: #e4e4e7;
+  margin-top: 1.25em;
+  margin-bottom: 0.5em;
+  line-height: 1.4;
+}
+
+.markdown-content h1 { font-size: 1.25rem; }
+.markdown-content h2 { font-size: 1.125rem; }
+.markdown-content h3 { font-size: 1rem; }
+
+.markdown-content p {
+  margin-top: 0.625em;
+  margin-bottom: 0.625em;
+}
+
+.markdown-content ul,
+.markdown-content ol {
+  margin-top: 0.5em;
+  margin-bottom: 0.5em;
+  padding-left: 1.5em;
+}
+
+.markdown-content ul { list-style-type: disc; }
+.markdown-content ol { list-style-type: decimal; }
+
+.markdown-content li {
+  margin-top: 0.25em;
+  margin-bottom: 0.25em;
+}
+
+.markdown-content li > p {
+  margin-top: 0.25em;
+  margin-bottom: 0.25em;
+}
+
+.markdown-content blockquote {
+  border-left: 3px solid #3f3f46;
+  padding-left: 1em;
+  color: #a1a1aa;
+  margin-top: 0.75em;
+  margin-bottom: 0.75em;
+}
+
+.markdown-content table {
+  width: 100%;
+  border-collapse: collapse;
+  margin-top: 0.75em;
+  margin-bottom: 0.75em;
+  font-size: 0.875rem;
+}
+
+.markdown-content th,
+.markdown-content td {
+  border: 1px solid #27272a;
+  padding: 0.5em 0.75em;
+  text-align: left;
+}
+
+.markdown-content th {
+  background: #18181b;
+  font-weight: 600;
+  color: #d4d4d8;
+}
+
+.markdown-content hr {
+  border: none;
+  border-top: 1px solid #27272a;
+  margin: 1.5em 0;
+}
+
+.markdown-content strong {
+  font-weight: 600;
+  color: #e4e4e7;
+}
+
+.markdown-content em {
+  font-style: italic;
+}
+
+.markdown-content img {
+  max-width: 100%;
+  border-radius: 0.5rem;
+}