feat(markdown-editor): implement wysiswyg markdown editor with live preview

- Build Go backend with Cobra CLI and REST API
  - CRUD operations for markdown files (GET, POST, PUT, DELETE)
  - File storage with flat .md file structure
  - Comprehensive logrus logging with JSON format
  - Static asset serving for frontend

- Build React/TypeScript frontend with Tailwind CSS
  - Markdown editor with live GFM preview
  - File management UI (list, create, open, delete)
  - Theme system (Dark/Light/System) with persistence
  - Responsive design (320px mobile, 1920px desktop)

- Add comprehensive test coverage
  - Backend: API, storage, and logger tests (13 tests passing)
  - Frontend: Editor and App component tests

- Setup Nix development environment with Go, Node.js, and TypeScript

shell.nix

@@ -0,0 +1,39 @@
+{
+  description = "Development environment for Markdown Editor";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    { self
+    , nixpkgs
+    , flake-utils
+    ,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = (
+          import nixpkgs {
+            system = system;
+          }
+        );
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            go
+            gopls
+            golangci-lint
+
+            nodejs
+            eslint
+            gnumake
+            lsof
+          ];
+        };
+      }
+    );
+}