fix(views): apply the starred default view on first resolve, for feed AND plex (S2)

The feed applied its starred default via a localStorage mirror
(LS.defaultViewFilters) written by SavedViewsProvider and read by
FeedFiltersProvider on load. That raced the login effect: on a genuinely
fresh browser (no LS.filters and no mirror yet) the feed could seed
DEFAULT_FILTERS and never re-apply the default until a reload. Plex had no
default-application at all.

Replace the mirror with a single path: SavedViewsProvider applies the
starred default via cfg.setFilters on the views query's FIRST resolve, but
only into a genuinely fresh module (no stored filters for the account, and
— feed only — no URL filter override). Applying persists it, so it happens
once and the last-view-wins path takes over, matching the documented
storage semantics. Because it runs off cfg.setFilters it delivers the Plex
port for free. Extract pickDefaultToSeed (pure, unit-tested) and cover the
first-resolve/stored/no-default cases for both modules in a provider test.

Removes the now-dead LS.defaultViewFilters key and its mirror read.

frontend/src/lib/storage.ts

@@ -14,9 +14,6 @@ export const LS = {
   hints: "siftlode.hints",
   lang: "siftlode.lang",
   filters: "siftlode.filters",
-  // Mirror of the default saved view's filters, so it can be applied synchronously on load
-  // (before the async saved-views query resolves). Kept in sync by SavedViewsWidget.
-  defaultViewFilters: "siftlode.defaultViewFilters",
   page: "siftlode.page",
   perfMode: "siftlode.perfMode",
   // The feed's view mode (see lib/feedView.ts) — a cache of the server pref, so a reload of an