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/savedViews.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { preferSticky } from "./savedViews";
+import { pickDefaultToSeed, preferSticky } from "./savedViews";
 
 const v = (id: number) => ({ id, name: `v${id}` });
 
@@ -24,3 +24,24 @@ describe("preferSticky", () => {
     expect(preferSticky([v(1), v(2)], null)?.id).toBe(1);
   });
 });
+
+describe("pickDefaultToSeed", () => {
+  const dv = (id: number, is_default = false) => ({ id, name: `v${id}`, is_default });
+
+  it("returns the starred default when the module is a fresh start", () => {
+    expect(pickDefaultToSeed([dv(1), dv(2, true)], true)?.id).toBe(2);
+  });
+
+  it("returns null when the module already has stored filters / a URL override (canSeed=false)", () => {
+    expect(pickDefaultToSeed([dv(1), dv(2, true)], false)).toBeNull();
+  });
+
+  it("returns null when there is no starred default, even on a fresh start", () => {
+    expect(pickDefaultToSeed([dv(1), dv(2)], true)).toBeNull();
+    expect(pickDefaultToSeed([], true)).toBeNull();
+  });
+
+  it("picks the first default deterministically if several are somehow flagged", () => {
+    expect(pickDefaultToSeed([dv(1, true), dv(2, true)], true)?.id).toBe(1);
+  });
+});