test(e4): readSelectedId ?pl guard, useUrlMirror dep-keying, usePref flush

The remaining E4 deep-review deferred gaps, in one sprint:
- readSelectedId split into playlistsInit.ts (react-refresh-clean, testable) —
  ?pl accepted only as a positive integer; empty/0/negative/fractional/non-numeric
  fall back to the stored selection; null when neither is present.
- useUrlMirror: mounts write the param, a same-serialized rerender does NOT
  re-fire (stable dep), a null value clears the key, other params preserved.
- usePref: a pending debounced write is FLUSHED on unmount (not dropped); an
  unmount with nothing pending stays silent.

20 new test cases; full FE gate green (tsc/eslint/prettier/knip, 309 vitest).

frontend/src/components/PlaylistsProvider.tsx

@@ -1,8 +1,9 @@
 import { createContext, useCallback, useContext, useMemo, useState, type ReactNode } from "react";
-import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
+import { LS, setAccountRaw } from "../lib/storage";
 import { useAccountRehydrate } from "../lib/useAccountRehydrate";
 import { useUrlMirror } from "../lib/useUrlMirror";
 import { useNavigation } from "./NavigationProvider";
+import { readSelectedId } from "./playlistsInit";
 
 // Owns the Playlists module's shared local state — the rail's name filter and the selected playlist
 // — lifted out of the App god component so the header search box, the PlaylistsRail, and the
@@ -38,17 +39,6 @@ export function usePlaylistsActions(): PlaylistsActions {
   return useContext(ActionsContext);
 }
 
-function readSelectedId(): number | null {
-  // A ?pl bookmark wins over the stored selection so a pasted link opens that playlist. Accept only a
-  // positive integer id — a bare "?pl=" gives Number("")===0 (finite!) and "?pl=1.5" a fractional id,
-  // both of which would select a non-existent playlist.
-  const fromUrl = new URLSearchParams(window.location.search).get("pl");
-  const n = fromUrl ? Number(fromUrl) : NaN;
-  if (Number.isInteger(n) && n > 0) return n;
-  const s = getAccountRaw(LS.playlist);
-  return s ? Number(s) : null;
-}
-
 export function PlaylistsProvider({ children }: { children: ReactNode }) {
   const [search, setSearchState] = useState("");
   const [selectedId, setSelectedIdState] = useState<number | null>(readSelectedId);

frontend/src/components/playlistsInit.test.ts

@@ -0,0 +1,54 @@
+// @vitest-environment jsdom
+//
+// readSelectedId: the Playlists selected-id seed. Backfills the E4 deep-review's deferred ?pl guard
+// coverage — a ?pl bookmark wins over the stored selection but only when it's a positive integer;
+// anything else (empty / zero / negative / fractional / non-numeric) falls back to the stored id.
+import { describe, it, expect, beforeEach, vi } from "vitest";
+
+const { stored } = vi.hoisted(() => ({ stored: { v: null as string | null } }));
+vi.mock("../lib/storage", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../lib/storage")>();
+  return { ...actual, getAccountRaw: () => stored.v };
+});
+
+import { readSelectedId } from "./playlistsInit";
+
+const setUrl = (search: string) => window.history.replaceState(null, "", search || "/");
+
+beforeEach(() => {
+  setUrl("/");
+  stored.v = null;
+});
+
+describe("readSelectedId ?pl guard", () => {
+  it("accepts a positive-integer ?pl", () => {
+    setUrl("?pl=5");
+    expect(readSelectedId()).toBe(5);
+  });
+
+  it("a ?pl bookmark wins over the stored selection", () => {
+    setUrl("?pl=5");
+    stored.v = "7";
+    expect(readSelectedId()).toBe(5);
+  });
+
+  it.each(["", "0", "-2", "1.5", "abc"])("rejects ?pl=%s and falls back to stored", (raw) => {
+    setUrl(`?pl=${raw}`);
+    stored.v = "7";
+    expect(readSelectedId()).toBe(7);
+  });
+
+  it("rejects a bad ?pl and returns null when there is no stored selection", () => {
+    setUrl("?pl=0");
+    expect(readSelectedId()).toBeNull();
+  });
+
+  it("uses the stored selection when there is no ?pl", () => {
+    stored.v = "9";
+    expect(readSelectedId()).toBe(9);
+  });
+
+  it("returns null when neither ?pl nor a stored selection is present", () => {
+    expect(readSelectedId()).toBeNull();
+  });
+});

frontend/src/components/playlistsInit.ts

@@ -0,0 +1,14 @@
+import { getAccountRaw, LS } from "../lib/storage";
+
+// The Playlists module's selected-id seed, split out of PlaylistsProvider so its ?pl guard is
+// unit-testable and the provider stays react-refresh-clean (component-only exports).
+export function readSelectedId(): number | null {
+  // A ?pl bookmark wins over the stored selection so a pasted link opens that playlist. Accept only a
+  // positive integer id — a bare "?pl=" gives Number("")===0 (finite!) and "?pl=1.5" a fractional id,
+  // both of which would select a non-existent playlist.
+  const fromUrl = new URLSearchParams(window.location.search).get("pl");
+  const n = fromUrl ? Number(fromUrl) : NaN;
+  if (Number.isInteger(n) && n > 0) return n;
+  const s = getAccountRaw(LS.playlist);
+  return s ? Number(s) : null;
+}