feat: code-screenshot template with syntax highlighting

[ajianaz]

Demand source: carbon.now.sh (36k+ GitHub stars) proves this is the most sought-after developer category. Cosy has snippet-* templates (mono, single color) but no highlighting.

Proposed design:

1. code-screenshot template (1200x750 default): window chrome (traffic lights), filename header, code area.
2. Simple server-side syntax highlighting in Rust (without a tree-sitter dependency if possible): regex-based tokenizer for keyword/string/comment/number/function per language (Go, Rust, TS, Python, Bash). Emit colored tspans (Catppuccin theme + light).
3. Fields: code (markup disabled, preserve newline/tab), language (for the tokenizer), filename, theme.
4. Tab-to-space expansion, optional line numbers.

Acceptance:

• Renders Go/Rust/TS snippets with a different color per token class.
• No heavy new dependency (tree-sitter optional behind a flag).
• Line counts > 20 still fit (auto font-size or scroll cut).

Effort: M (1-2 days). Priority #2.