feat(library): exclude chosen folders from music browsing

Replaces `src/lib/utils/podcastFilter.ts` — a shipped personal workaround
that dropped any item whose name, album, album artist or artist was
literally "Podcasts" — with a real user setting applied in Rust.

The old filter was wrong twice over: it hardcoded one user's folder
layout keyed on an English literal, and it put a domain rule (what a
query should return) in the presentation layer. It slipped past
`check:boundary` only because it matched on names rather than on an
item-type array.

- `repository::exclusions` owns the rule and the process-wide id set,
  the same shape as `online::STREAMING_QUALITY` so it survives a
  repository being rebuilt on re-login.
- `HybridRepository` applies it where the cache and server legs of every
  cache-first query converge (`parallel_race` / `race_with_refresh`),
  plus the bespoke `get_items` path and the server-only reads. Filtering
  before the "has content" check is what makes a cache page of nothing
  but hidden items fall through to the server.
- Exclusion is by stable item id, never by name, and matches an item's
  own id or any container link it carries (parent, album, library,
  series, season, artist).
- A direct `get_item` lookup and the Downloads surface are deliberately
  unfiltered: hiding those would break playback and file management of
  anything inside a hidden folder.
- `LibrarySettings` persists to `app_settings` and is restored in the
  setup hook, alongside the streaming-quality cap. Default is an empty
  list — nobody inherits the old "Podcasts" behaviour.
- New commands `library_get_settings`, `library_set_settings` and
  `library_get_exclusion_candidates`; the candidates read goes through
  `get_items_unfiltered` so an already-hidden folder still appears in the
  picker and the setting can be undone.
- Settings page gains a "Hidden Folders" section that renders the
  backend's candidate list and sends back ticked ids; it decides nothing.

TRACES: UR-076 | DR-209 | UT-203

src/lib/utils/podcastFilter.ts

@@ -1,30 +0,0 @@
-// HACK: hide "Podcasts" from the music library.
-//
-// The user stores podcasts inside the music library under a folder/album named
-// "Podcasts", so they leak into album/artist/track/playlist queries. Jellyfin's
-// item queries here don't give us a clean server-side exclusion for that folder,
-// so we filter client-side by name. This is intentionally a blunt instrument:
-// anything whose own name, album, or (album) artist is literally "Podcasts" is
-// dropped. If the folder is ever renamed, update PODCAST_FOLDER_NAME.
-import type { MediaItem } from "$lib/api/types";
-
-const PODCAST_FOLDER_NAME = "podcasts";
-
-function isPodcastName(value: string | null | undefined): boolean {
-  return value?.trim().toLowerCase() === PODCAST_FOLDER_NAME;
-}
-
-/** True when an item belongs to the "Podcasts" folder/album and should be hidden. */
-export function isPodcastItem(item: MediaItem): boolean {
-  return (
-    isPodcastName(item.name) ||
-    isPodcastName(item.albumName) ||
-    isPodcastName(item.albumArtist) ||
-    (item.artists?.some(isPodcastName) ?? false)
-  );
-}
-
-/** Remove "Podcasts" entries from a list of music items. */
-export function excludePodcasts(items: MediaItem[]): MediaItem[] {
-  return items.filter((item) => !isPodcastItem(item));
-}