feat(library): exclude chosen folders from music browsing

Replaces a hardcoded filter that dropped anything named "Podcasts" from music
results — one user's library layout compiled into the shipped product, keyed on
an English literal, applied only at the six call sites someone had remembered.

Exclusion is now a user setting stored in Rust and applied at the repository
layer's convergence points, so scope is decided once and is the same on every
screen. It matches on folder id rather than name: a title is not what an item
is, which is why an album legitimately called "Podcasts" used to vanish.

Deliberately not filtered: get_item (an id asked for by name was navigated to on
purpose, and refusing it would break playback of anything inside a hidden
folder), get_downloaded_items (hiding a download would leave the user unable to
delete a file whose disk usage they can still see), and the offline cache (an
exclusion is a view preference and must be reversible without a re-crawl).

Also removes src/lib/utils/validation.ts — six exported validators with no
caller outside their own test file, which made the module read as covered
input validation while guarding 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 "$lib/utils/podcastFilter";
   import { createLogger } from "$lib/utils/logger";
 
   const log = createLogger("GenericMediaListPage");
@@ -87,7 +86,7 @@
     unlistenSearch = await listen<SearchUpdateEvent>("search-event", (event) => {
       const { requestId, result } = event.payload;
       if (requestId !== searchRequestId) return;
-      items = excludePodcasts(result.items);
+      items = result.items;
     });
   }
 
@@ -121,8 +120,9 @@
       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,
@@ -139,7 +139,7 @@
         );
         // 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.
@@ -154,7 +154,7 @@
           // resolves to online vs offline. TRACES: UR-067 | DR-116
           favoritesOnly: favoritesOnly ? true : undefined,
         });
-        items = excludePodcasts(result.items);
+        items = result.items;
       }
     } catch (e) {
       log.error(`Failed to load ${config.itemType}:`, e);