feat: polished dark_dev developer blog theme sample (#24)

[lb1192176991-lab]

What

Replaced the bare-bones dark_dev.html (898 bytes, minimal styling) with a polished, terminal-inspired developer blog theme featuring:

• Monospace font stack (SF Mono / Cascadia Code / JetBrains Mono)
• Dot-grid background pattern
• Syntax-highlighted code blocks (Go, Rust, TypeScript, Python, CSS)
• Responsive two-column layout (posts + sidebar) that collapses on mobile
• Tag badges with language-specific colors
• Navigation bar with active state
• Proper CSS custom properties for easy theming

Why

The existing dark_dev.html was a stub with minimal CSS and no developer aesthetic. This theme delivers a complete, screenshot-ready dark developer blog that matches the dark_dev style name — terminal-inspired, code-first, and visually distinct from the other samples.

Testing

• HTML validates as self-contained (all CSS inline, no external dependencies)
• Responsive layout tested at mobile (320px) and desktop (1440px) breakpoints
• All sample content renders correctly with proper typography and spacing
• Code blocks display with syntax highlighting classes

Closes #24