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

package.json

@@ -33,6 +33,14 @@
 		"vitest": "^3.1.3"
 	},
 	"dependencies": {
+		"@codemirror/autocomplete": "^6.20.0",
+		"@codemirror/commands": "^6.10.1",
+		"@codemirror/lang-css": "^6.3.1",
+		"@codemirror/lang-html": "^6.4.11",
+		"@codemirror/state": "^6.5.2",
+		"@codemirror/view": "^6.39.4",
+		"@emmetio/codemirror6-plugin": "^0.4.0",
+		"codemirror": "^6.0.2",
 		"whatwg-fetch": "^3.6.20"
 	}
 }