Display custom recipe metadata (e.g. nutrition) on the recipe page #459

Problem

Cooklang's YAML frontmatter supports arbitrary custom metadata keys beyond the standard ones (servings, time, course, ...), but CookCLI's web UI only shows a handful of known fields as pills, plus flat scalar custom fields as key: value pills. There's no good way to show structured custom metadata — the most common example being nutrition facts (calories, protein, fat, ...) that many recipe collections carry per recipe.

Proposal

Any non-standard YAML key whose value is a list or a mapping should render as its own line on the recipe page, below the tags — one line per "family":

nutrition:
  kcal: 234
  proteins: 9.8
  lipids: 17.6
allergens:
  - gluten
  - tree nuts

• Works generically for any such key with no code changes — allergens: above should just work.
• A couple of well-known families (nutrition, and file metadata like created-by/modified-at) could get nicer treatment: matching icons, and translated field names via the existing i18n system.
• The recipe list page could show a compact calorie badge next to the tags when nutrition is present, without cluttering the cards with the rest of the nutrition breakdown.
• A way to hide a family or a single entry from display (e.g. a .-prefixed key) would help for metadata that's useful to keep in the file but not show on the page.

Proposed implementation

Opened as #458, which implements the above (generic rendering for any family, specific icon/i18n treatment for nutrition and file/meta, .-prefix hiding, kcal badge on the list page).