feat: display nutrition data from metadata

[romainhild]

Goal

Display nutrition information stored in .cook recipe metadata in the web UI, when available. The info are in a collapsible section between the metadata pill and the recipe steps.
Two storage formats exist: flat top-level keys (hand-authored) and a nested nutrition: mapping (from imported recipes).
Display only if at least one information is available.
Display the metadata as-is, no checks, no scalings, no fetching data.

Scope

• No changes to cooklang-rs — nutrition is not a StdKey; we read it from the raw metadata map
• No changes to the CLI recipe command output — this is UI-only
• No per-serving scaling of nutrition values — values are displayed as stored
• Both builder instances in builders.rs updated for consistency

Out of scope

• Nutrition per-serving math when scaling
• Editing nutrition from the UI
• CLI text/JSON output of nutrition

Specs and implementation plans have been added in docs/

Disclaimer: The implementation has been done using Claude Code. I made the designs decisions and have reviewed the code.