feat: add CodeMirror 6 editor with Emmet support

- Replace textarea with CodeMirror 6 for syntax highlighting
- Add Emmet abbreviation expansion (Tab to expand)
- Support HTML and CSS language modes with autocomplete
- Add dark theme matching app design
- Tab indentation with Shift-Tab for outdent
- Update help modal with Emmet shortcuts

src/main.css

@@ -264,9 +264,23 @@ code, kbd {
 .editor-content {
 	flex: 1;
 	display: flex;
+	flex-direction: column;
 	min-height: 0;
+	background: var(--editor-bg);
+	overflow: hidden;
 }
 
+/* CodeMirror container styles */
+.editor-content .cm-editor {
+	flex: 1;
+	height: 100%;
+}
+
+.editor-content .cm-scroller {
+	overflow: auto;
+}
+
+/* Legacy textarea (fallback) */
 .code-input {
 	flex: 1;
 	width: 100%;