Replace textarea with CodeMirror 6 editor

- Add CodeMirror 6 with oneDark theme and markdown language support
- Bundle via esbuild into public/vendor/ (built on deploy, not committed)
- Add codemirror-entry.mjs as bundle entry point
- Fix SPA fallback to only serve index.html for /, not all paths
- Fix mime type handling by mutating mime-types registry
- Add Write/Preview tabs; preview renders on tab switch only
- Multi-cursor via Shift+Alt+Up/Down; Alt+Click adds cursor
- gitignore package-lock.json and public/vendor/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -9,7 +9,17 @@
 		"start": "node server.mjs"
 	},
 	"dependencies": {
+		"@codemirror/commands": "^6.10.3",
+		"@codemirror/lang-markdown": "^6.5.0",
+		"@codemirror/language": "^6.12.3",
+		"@codemirror/state": "^6.6.0",
+		"@codemirror/theme-one-dark": "^6.1.3",
+		"@codemirror/view": "^6.43.0",
+		"@lezer/highlight": "^1.2.3",
 		"express": "^5.2.1",
 		"yaml-js": "^0.3.1"
+	},
+	"devDependencies": {
+		"esbuild": "^0.28.0"
 	}
 }