Replace per-field Option<String> with Option<StringOrList> for all string
fields in the Frontmatter struct (owner, cadence, status, icon, color,
sidebar_label, template, sort, view, trashed_at, created_at, created_time).
Previously, fields like Owner stored as YAML arrays (e.g. `Owner: [Luca]`)
caused serde to fail the entire Frontmatter deserialization, defaulting all
fields to None — including is_a, which broke the type badge display.
Now all string fields use StringOrList with into_scalar() normalization:
- Single-element array → unwrap to scalar
- Multi-element array → take first element
- Scalar → unchanged
- Empty array → None
- Absent → None
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
6371e80f06