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-tauri/src/settings.rs

@@ -320,6 +320,52 @@ impl VideoSettings {
     }
 }
 
+/// Library browsing preferences.
+///
+/// Currently a single list: the folders (or whole libraries) the user has asked
+/// to keep out of browsing. It is a *list of ids*, never names — names are
+/// unstable, locale-dependent and non-unique, and the hardcoded name filter this
+/// setting replaced broke on exactly that. What the ids then hide is decided in
+/// `repository::exclusions`; this struct is only how the choice is carried and
+/// persisted.
+///
+/// The default is an empty list: nobody inherits another user's folder layout.
+///
+/// TRACES: UR-076 | DR-209
+#[derive(specta::Type, Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct LibrarySettings {
+    /// Stable item ids of the folders/libraries hidden from browsing.
+    ///
+    /// `#[serde(default)]` so settings JSON persisted before this field existed
+    /// loads as the previous behaviour (nothing hidden).
+    #[serde(default)]
+    pub excluded_item_ids: Vec<String>,
+}
+
+impl LibrarySettings {
+    /// Drop blanks and duplicates from the id list.
+    ///
+    /// Applied on the way in from IPC and on the way out of the database, so a
+    /// hand-edited or half-written value cannot make the list grow without bound
+    /// or carry an empty id (which would match nothing but still be shown as a
+    /// selection in the picker).
+    ///
+    /// TRACES: UR-076 | DR-209
+    pub fn sanitised(mut self) -> Self {
+        let mut seen: Vec<String> = Vec::with_capacity(self.excluded_item_ids.len());
+        for id in self.excluded_item_ids.drain(..) {
+            let id = id.trim().to_string();
+            if id.is_empty() || seen.contains(&id) {
+                continue;
+            }
+            seen.push(id);
+        }
+        self.excluded_item_ids = seen;
+        self
+    }
+}
+
 /// Serialise `AudioSettings` into the JSON payload handed to the Android player
 /// over JNI.
 ///