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/components/library/GenericMediaListPage.svelte

@@ -19,7 +19,6 @@
   import LibraryGrid from "./LibraryGrid.svelte";
   import TrackList from "./TrackList.svelte";
   import AlphabetScrollBar from "./AlphabetScrollBar.svelte";
-  import { excludePodcasts } from "$lib/utils/podcastFilter";
 
   /**
    * Generic media list page supporting Albums, Artists, Playlists, and Tracks
@@ -84,7 +83,7 @@
     unlistenSearch = await listen<SearchUpdateEvent>("search-event", (event) => {
       const { requestId, result } = event.payload;
       if (requestId !== searchRequestId) return;
-      items = excludePodcasts(result.items);
+      items = result.items;
     });
   }
 
@@ -118,8 +117,9 @@
       if (items.length === 0) loading = true;
       const repo = auth.getRepository();
 
-      // Use backend search if search query is provided, otherwise use getItems with sort
-      // HACK: excludePodcasts drops the "Podcasts" folder stored in the music library.
+      // Use backend search if search query is provided, otherwise use getItems
+      // with sort. Neither result is filtered here: folders the user chose to
+      // hide are dropped by the repository layer. TRACES: UR-076 | DR-209
       if (debouncedSearchQuery.trim()) {
         // Phase 1: instant cache-only (downloaded) results. The merged
         // cache+server union arrives later via the `search-event` listener,
@@ -136,7 +136,7 @@
         );
         // Only apply if this is still the active query.
         if (requestId === searchRequestId) {
-          items = excludePodcasts(result.items);
+          items = result.items;
         }
       } else {
         // Leaving search — invalidate any in-flight server results.
@@ -151,7 +151,7 @@
           // resolves to online vs offline. TRACES: UR-067 | DR-116
           favoritesOnly: favoritesOnly ? true : undefined,
         });
-        items = excludePodcasts(result.items);
+        items = result.items;
       }
     } catch (e) {
       console.error(`Failed to load ${config.itemType}:`, e);