Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/launch-editor/index.d.ts

@@ -0,0 +1,16 @@
+/**
+ * Launch an editor to open a file at a specific line and column.
+ *
+ * @param file File path with optional line and column numbers (e.g.
+ *   "/path/to/file.js:10:2").
+ * @param specifiedEditor Optional editor command or path to use. Will be
+ *   parsed using `shell-quote`.
+ * @param onErrorCallback Optional callback for handling errors.
+ */
+declare function launchEditor(
+  file: string,
+  specifiedEditor?: string | ((fileName: string, errorMessage: string | null) => void),
+  onErrorCallback?: (fileName: string, errorMessage: string | null) => void
+): void;
+
+export = launchEditor;