[UI] Show coverage badges for demo vs full terminology schemes

Problem summary

ICD-11, LOINC, and CPT ship as curated demo subsets (~64–70 records each) while ICD-10 and ATC-5 are full distributions. The UI does not distinguish demo vs full coverage, which can mislead users.

Affected area

• app/index.tsx, app/components/SchemeTabs.tsx
• data/vocabularies/source-metadata.json
• app/services/sourceMetadata.ts

Expected behavior

Each scheme shows a clear coverage indicator (e.g. "Demo · 70 codes" vs "Full · 74,260 codes") and last-updated metadata where available.

Actual behavior

All schemes appear equivalent in the tab UI. README notes demo subsets but in-app UX does not surface this.

Acceptance criteria

• Active scheme displays record count and coverage tier from source-metadata.json.
• Demo schemes show a visible "Demo subset" badge or equivalent.
• Copy is localized via i18n where applicable.
• About screen links to data provenance for the active scheme.

Start with app/index.tsx and app/components/SchemeTabs.tsx, then inspect app/services/sourceMetadata.ts and data/vocabularies/source-metadata.json to understand the available metadata. Trace how the active scheme and About screen are rendered. Done means each scheme shows its coverage tier and record count, applicable last-updated and provenance details appear, demo schemes are visibly marked, and copy is localized where supported.