feat: code-screenshot template with syntax highlighting

[ajianaz]

Sumber demand: carbon.now.sh (36k+ GitHub stars) membuktikan kategori paling dicari developer. Cosy punya snippet-* (mono, 1 warna) tapi tanpa highlight.

Desain proposal:

1. Template code-screenshot (1200x750 default): window chrome (traffic light), filename header, code area.
2. Syntax highlighting sederhana server-side di Rust (tanpa dependency tree-sitter jika bisa): regex-based tokenizer untuk keyword/string/comment/number/function per bahasa (Go, Rust, TS, Python, Bash). Emit tspan berwarna (theme Catppuccin + light).
3. Field: code (markup disabled, preserve newline/tab), language (untuk tokenizer), filename, theme.
4. Tab-to-space expand, line numbers opsional.

Acceptance:

• Render snippet Go/Rust/TS dengan warna berbeda per token class.
• Tidak ada dependency baru berat (tree-sitter optional di belakang flag).
• Line count > 20 tetap muat (auto font-size atau scroll cut).

Effort: M (1-2 hari). Prioritas #2.