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/stores/music.ts

@@ -4,7 +4,6 @@
 import { writable, derived } from "svelte/store";
 import type { MediaItem, Genre } from "$lib/api/types";
 import { auth } from "./auth";
-import { excludePodcasts } from "$lib/utils/podcastFilter";
 import { selectDiverseGenres, sampleAcross } from "$lib/utils/genreDiversity";
 import { buildHeroMix } from "$lib/utils/heroMix";
 
@@ -97,23 +96,20 @@ function createMusicStore() {
           .catch(() => [] as MediaItem[]),
       ]);
 
-      // HACK: drop the "Podcasts" folder that lives inside the music library.
-      const recentlyPlayedAlbums = excludePodcasts(recentlyPlayed);
-      const newlyAddedAlbums = excludePodcasts(newlyAdded.items);
-      const playlistItems = excludePodcasts(playlistsResult.items);
-      const rediscoverAlbums = excludePodcasts(rediscover);
-      const surpriseAlbums = excludePodcasts(surprise);
+      // Nothing is filtered here: folders the user chose to hide are already
+      // gone, dropped by the repository layer that answered these queries.
+      // TRACES: UR-076 | DR-209
 
       // Mix the hero: fresh-in-your-ears first, then "remember this?", then
       // random albums from across the library.
-      const heroItems = buildHeroMix([recentlyPlayedAlbums, rediscoverAlbums, surpriseAlbums], hasArt);
+      const heroItems = buildHeroMix([recentlyPlayed, rediscover, surprise], hasArt);
 
       update(s => ({
         ...s,
-        recentlyPlayed: recentlyPlayedAlbums,
-        newlyAdded: newlyAddedAlbums,
-        playlists: playlistItems,
-        rediscover: rediscoverAlbums,
+        recentlyPlayed,
+        newlyAdded: newlyAdded.items,
+        playlists: playlistsResult.items,
+        rediscover,
         heroItems,
         isLoading: false,
       }));
@@ -140,8 +136,7 @@ function createMusicStore() {
         recursive: true,
         limit: SECTION_LIMIT,
       });
-      // HACK: drop the "Podcasts" folder that lives in the music library.
-      return { id: genre.id, name: genre.name, items: excludePodcasts(result.items) };
+      return { id: genre.id, name: genre.name, items: result.items };
     } catch (e) {
       console.warn(`Failed to load genre row "${genre.name}":`, e);
       return { id: genre.id, name: genre.name, items: [] };