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
