Mirror the Plex-browse content state into the address bar so a browse view is
bookmarkable/shareable, the Plex analogue of the feed's urlState. Keys are
NAMESPACED (pscope/psort/pshow/pf) rather than reusing the feed's bare
scope/sort/show: FeedFiltersProvider actively strips its own KEYS from the URL
whenever off the feed page, so a shared key would be deleted out from under the
Plex page. The namespaced keys are Plex-unique (like ?v=/?pl), so no other
provider ever reads or clears them.
- New lib/plexUrlState.ts: plexParamUpdates / plexParamsToState / hasPlexParams.
The pf filter blob reuses the SAME appendPlexFilters wire serializer (new
public plexFiltersToParams + inverse parsePlexFilters, kept next to each other
so a new wire key can't be added on one side only).
- PlexProvider: URL-seed wins over localStorage at mount; the account-pin
rehydrate persists the seed instead of clobbering it (mirrors feed's
initialHadUrlFilters); useUrlMirror mirrors/clears the 4 keys, gated on
page===plex. q/field stay out of the shareable contract.
- NavigationProvider: a page switch now also nulls the 4 plex keys.
Tests: 8 new round-trip cases in plexUrlState.dom.test.ts (FE vitest 483→491).
