feat(portal): display dataset conformsTo schema metadata

[BatLeDev]

Companion to data-fair#383 (introduces the conformsTo dataset metadata: external schema reference with title, version, url).

• Adds an optional conformsTo field on the portal-side Dataset type (api/types/index.ts).
• Renders the schema reference in the dataset metadata card (portal/app/components/dataset/dataset-metadata.vue), placed after the frequency row to match the order of metadata in the data-fair org settings:
  • Title displayed as a link to url (target="_blank", rel="noopener", simple-link style). Falls back to plain text when there is no URL; falls back to the URL itself as link label when there is no title.
  • Version shown as a small secondary v-chip to the right.
  • Row is hidden when conformsTo exists but all three sub-fields are empty.
• Label is resolved through the existing metadata-settings endpoint (metadataLabel('conformsTo')), so the per-org custom title configured in data-fair (Paramètres → Qualité → Métadonnées → Schéma) is honored. Default i18n label: "Schema:" / "Schéma :".
• BaseMetadataSettings extended with the conformsTo key so the typed label lookup compiles.