feat: add Web UI artist detail, archive, and ignore actions

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.

docs/plans/2026-07-19-notifier-webui-sync.md

@@ -125,11 +125,11 @@ name collisions.)
 - [x] run tests - must pass before task 8
 
 ### Task 8: Web UI — artist detail + archive + ignore actions
-- [ ] artist page: local albums (Subsonic) + found missing (MB cache) + ignore buttons
-- [ ] archive page: `GetIgnoredReleases` with restore action
-- [ ] POST handlers: `SetReleaseIgnored(rgid, true/false)`; "ignore all singles of artist" toggles `artist_settings.ignore_singles`
-- [ ] write tests: ignore sets flag + removes from dashboard missing; restore clears flag; auth enforced on POST
-- [ ] run tests - must pass before task 9
+- [x] artist page: local albums (Subsonic) + found missing (MB cache) + ignore buttons
+- [x] archive page: `GetIgnoredReleases` with restore action
+- [x] POST handlers: `SetReleaseIgnored(rgid, true/false)`; "ignore all singles of artist" toggles `artist_settings.ignore_singles`
+- [x] write tests: ignore sets flag + removes from dashboard missing; restore clears flag; auth enforced on POST
+- [x] run tests - must pass before task 9
 
 ### Task 9: Verify acceptance criteria
 - [ ] run full suite `go test ./...` — all pass