feat: replace textarea with Lexical WYSIWYG markdown composer

Replace the plain <textarea> in ChatPane with a full WYSIWYG
markdown composer built on Lexical 0.42.0.

Composer features:
- Rich text editing with inline formatting (bold, italic, code)
- Block-level elements: headings, bullet/numbered lists, code blocks,
  blockquotes, links via markdown shortcuts (e.g. ## , - , \\\)
- Formatting toolbar with active-state indicators
- Markdown paste auto-import when the editor is empty
- Enter to send, Shift+Enter for newline (matches prior behavior)
- Disabled state syncs with session busy/config states
- Serializes to markdown on submit via the backend helpers

Files:
- New: MarkdownComposer.tsx (Lexical editor, toolbar, internal plugins)
- Modified: ChatPane.tsx (swap textarea for MarkdownComposer, simplify state)
- Modified: styles.css (add markdown composer theme classes)

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

src/renderer/styles.css

@@ -140,6 +140,112 @@ textarea {
   max-height: 200px;
 }
 
+/* ── Markdown composer (Lexical) ────────────────────────── */
+
+.markdown-composer-content {
+  position: relative;
+}
+
+.markdown-composer-editable {
+  min-height: 40px;
+  max-height: 200px;
+  overflow-y: auto;
+  padding: 8px 48px 8px 16px;
+  font-size: 14px;
+  line-height: 1.6;
+  color: #f4f4f5;
+  outline: none;
+  cursor: text;
+}
+
+.markdown-composer-editable[contenteditable="false"] {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
+.markdown-composer-placeholder {
+  position: absolute;
+  top: 8px;
+  left: 16px;
+  font-size: 14px;
+  line-height: 1.6;
+  color: #52525b;
+  pointer-events: none;
+  user-select: none;
+}
+
+/* Paragraphs */
+.mc-p {
+  margin: 0;
+}
+
+/* Headings */
+.mc-h1, .mc-h2, .mc-h3 {
+  font-weight: 600;
+  margin: 0;
+  line-height: 1.4;
+}
+.mc-h1 { font-size: 1.1em; }
+.mc-h2 { font-size: 1.05em; }
+.mc-h3 { font-size: 1em; }
+
+/* Inline formatting */
+.mc-bold { font-weight: 600; }
+.mc-italic { font-style: italic; }
+.mc-strikethrough { text-decoration: line-through; }
+.mc-underline { text-decoration: underline; }
+.mc-inline-code {
+  background: rgba(63, 63, 70, 0.6);
+  border-radius: 3px;
+  padding: 1px 4px;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, monospace;
+  font-size: 0.9em;
+}
+
+/* Lists */
+.mc-ul {
+  list-style-type: disc;
+  padding-left: 1.5em;
+  margin: 2px 0;
+}
+.mc-ol {
+  list-style-type: decimal;
+  padding-left: 1.5em;
+  margin: 2px 0;
+}
+.mc-li {
+  margin: 1px 0;
+}
+
+/* Blockquote */
+.mc-blockquote {
+  border-left: 3px solid #3f3f46;
+  padding-left: 0.75em;
+  color: #a1a1aa;
+  margin: 2px 0;
+}
+
+/* Code block */
+.mc-code-block {
+  background: rgba(24, 24, 27, 0.8);
+  border: 1px solid #27272a;
+  border-radius: 6px;
+  padding: 8px 12px;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, monospace;
+  font-size: 0.9em;
+  white-space: pre-wrap;
+  tab-size: 2;
+  margin: 4px 0;
+  overflow-x: auto;
+}
+
+/* Link */
+.mc-link {
+  color: #818cf8;
+  text-decoration: underline;
+  text-underline-offset: 2px;
+}
+
 /* React Flow dark theme overrides */
 .react-flow__node {
   font-family: inherit;