feat(tui): add link picker and navigation history

Press [ in detail view to open link picker showing all [[links]]
in the current entry. Enter follows a link, resolving by title
then body substring. Navigation history stack enables esc to pop
back through followed links before returning to list.

Adds Store.ResolveLink() for non-transactional link resolution
from the TUI layer.

internal/db/links.go

@@ -55,6 +55,26 @@ func syncLinks(ctx context.Context, tx *sql.Tx, s *Store, entityID string, body
 	return nil
 }
 
+func (s *Store) ResolveLink(ctx context.Context, linkText string) (*Entity, error) {
+	lower := strings.ToLower(linkText)
+
+	var id string
+	err := s.db.QueryRowContext(ctx, `
+		SELECT id FROM entities
+		WHERE LOWER(title) = ? AND deleted_at IS NULL
+		ORDER BY created_at DESC LIMIT 1`, lower).Scan(&id)
+	if err != nil {
+		err = s.db.QueryRowContext(ctx, `
+			SELECT id FROM entities
+			WHERE LOWER(body) LIKE ? AND deleted_at IS NULL
+			ORDER BY created_at DESC LIMIT 1`, "%"+lower+"%").Scan(&id)
+	}
+	if err != nil {
+		return nil, ErrNotFound
+	}
+	return s.Get(ctx, id)
+}
+
 func (s *Store) LoadBacklinks(ctx context.Context, entityID string) ([]Backlink, error) {
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT e.id, e.title, e.body, el.link_text