feat(plex): port the saved-views (personal views) UI to Plex (frontend)

Plex now has the same personal-views system as the feed — the rail header icon
opens the quick-pick popover, the rail title opens the manager modal (list /
create / rename / delete / reorder / set-default / apply), and the active view
leads the chip row with a "modified" dot.

One SavedViewsProvider serves both modules: it switches `source` (feed ⟷ plex)
by the active page (both filter providers are global, so it reads whichever the
page owns) and runs the same logic on per-source wiring — the live filters,
apply, a stable signature (feed filtersToParams / plex plexFiltersSignature),
coercion (asFilters / coercePlexFilters), and what to strip on save. Moved below
PlexProvider in the tree so it can read plex state; resets sticky/modal on a
source switch. The shared popover/modal/chip are unchanged; the modal describes
a plex view's captured filters via a pure buildPlexActiveFilters (extracted from
usePlexActiveFilters — no duplication). api.savedViews/createView take a source;
`SavedView.filters` is now an opaque blob. New coercePlexFilters/plexFilters-
Signature (also dedups PlexProvider's collection migration). Empty-state copy
genericised (no longer says "feed"). A Plex view captures the rail filters (not
watch-state/sort/scope).

Verified end-to-end on localdev: create/apply/preview/default/delete on plex,
feed views stay separate + unregressed (402 vitest), no console errors.

frontend/src/lib/api.ts

@@ -51,7 +51,12 @@ import { adminApi } from "./api/admin";
 export type * from "./api/admin";
 import { plexApi } from "./api/plex";
 export type * from "./api/plex";
-export { EMPTY_PLEX_FILTERS, plexFilterCount } from "./api/plex";
+export {
+  EMPTY_PLEX_FILTERS,
+  plexFilterCount,
+  coercePlexFilters,
+  plexFiltersSignature,
+} from "./api/plex";
 import { messagingApi } from "./api/messaging";
 export type * from "./api/messaging";
 import { downloadsApi } from "./api/downloads";