feat: implement WYSIWYG markdown editor

Add complete markdown editor with Go backend and React/TypeScript frontend.

Backend:
- Cobra CLI with configurable host, port, data-dir, static-dir flags
- REST API for CRUD operations on markdown files (GET, POST, PUT, DELETE)
- File storage with flat .md structure
- Comprehensive Logrus logging for all operations
- Static asset serving for frontend

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Live markdown editor with GFM preview (react-markdown)
- File management UI (list, create, open, save, delete)
- Theme system (Light/Dark/System) with localStorage persistence
- Responsive design (320px - 1920px+)

Testing:
- 6 backend tests covering CRUD round-trip, validation, error handling
- 19 frontend tests covering API, theme system, and UI components
- All tests passing with single 'make test' command

Build:
- Frontend compiles to optimized assets in dist/
- Backend can serve frontend via --static-dir flag

frontend/.eslintrc.json

@@ -0,0 +1,18 @@
+{
+  "root": true,
+  "env": { "browser": true, "es2020": true },
+  "extends": [
+    "eslint:recommended",
+    "plugin:@typescript-eslint/recommended",
+    "plugin:react-hooks/recommended"
+  ],
+  "ignorePatterns": ["dist", ".eslintrc.json"],
+  "parser": "@typescript-eslint/parser",
+  "plugins": ["react-refresh"],
+  "rules": {
+    "react-refresh/only-export-components": [
+      "warn",
+      { "allowConstantExport": true }
+    ]
+  }
+}