feat: implement WYSIWYG markdown editor

Add full-stack markdown editor with Go backend and React frontend.

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- REST API for markdown file CRUD operations
- File storage with flat directory structure
- logrus logging for all operations
- Static file serving for frontend
- Comprehensive tests for CRUD and static assets

Frontend:
- React + TypeScript + Vite + Tailwind CSS
- Live markdown preview with marked (GFM)
- File management: list, create, open, save, delete
- Theme system: Dark/Light/System with persistence
- Responsive design (320px to 1920px)
- Component tests for Editor, Preview, Sidebar

Build:
- Makefile for build, test, and run automation
- Single command testing (make test)

Closes SPEC.md requirements

backend/go.sum

@@ -0,0 +1,14 @@
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+go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+golang.org/x/sys v0.13.0 h1:Af8nKPmuFypiUBjVoU9V20FiaFXOcuZI21p0ycVYYGE=
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