Task 8's ignore/restore/ignore-singles actions live on the artist detail
page, but the dashboard rendered artist names as plain text with no link,
making those actions unreachable through normal UI navigation. Wrap the
name in an anchor to /artist/{id} and assert the link in the dashboard test.
Implements Task 8: artist detail page (local albums + found-missing
with ignore buttons), ignored-releases archive with restore, and POST
handlers toggling ignore flags and ignore_singles. Adds ErrArtistNotFound
sentinel so callers can distinguish missing artists, and wires routes via
the enhanced ServeMux path wildcard.
