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/lib/usePref.dom.test.ts

@@ -155,6 +155,29 @@ describe("usePref", () => {
       expect(savePrefs).toHaveBeenCalledTimes(1);
       expect(savePrefs).toHaveBeenCalledWith({ vol: 30 });
     });
+
+    it("flushes a pending debounced write on unmount instead of dropping it", () => {
+      const qc = makeClient();
+      seedMe(qc, {});
+      const { result, unmount } = renderHook(() => usePref<number>("vol", 0, { debounceMs: 300 }), {
+        wrapper: wrapperFor(qc),
+      });
+      act(() => result.current[1](42));
+      expect(savePrefs).not.toHaveBeenCalled(); // still inside the debounce window
+      unmount(); // cleanup must flush the pending write, not lose it
+      expect(savePrefs).toHaveBeenCalledTimes(1);
+      expect(savePrefs).toHaveBeenCalledWith({ vol: 42 });
+    });
+
+    it("unmount with no pending write does not call savePrefs", () => {
+      const qc = makeClient();
+      seedMe(qc, {});
+      const { unmount } = renderHook(() => usePref<number>("vol", 0, { debounceMs: 300 }), {
+        wrapper: wrapperFor(qc),
+      });
+      unmount();
+      expect(savePrefs).not.toHaveBeenCalled();
+    });
   });
 });
 

frontend/src/lib/useUrlMirror.dom.test.ts

@@ -0,0 +1,51 @@
+// @vitest-environment jsdom
+//
+// useUrlMirror: mirror a set of nav params onto the CURRENT history entry (replaceState) whenever they
+// change. Backfills the E4 deep-review's deferred coverage — the effect keys on the SERIALIZED updates
+// so an inline object literal (new identity every render) only fires when a param actually changes; a
+// null value clears a key; and every other param on the URL is preserved.
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { renderHook } from "@testing-library/react";
+import { useUrlMirror } from "./useUrlMirror";
+
+const setUrl = (search: string) => window.history.replaceState(null, "", search || "/");
+let spy: ReturnType<typeof vi.spyOn>;
+
+beforeEach(() => {
+  setUrl("/");
+  spy = vi.spyOn(window.history, "replaceState");
+});
+afterEach(() => spy.mockRestore());
+
+describe("useUrlMirror", () => {
+  it("writes the param onto the current entry on mount", () => {
+    renderHook(() => useUrlMirror({ pl: "5" }));
+    expect(window.location.search).toBe("?pl=5");
+    expect(spy).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not re-fire when the serialized updates are unchanged (stable dep)", () => {
+    const { rerender } = renderHook((p: Record<string, string | null>) => useUrlMirror(p), {
+      initialProps: { pl: "5" } as Record<string, string | null>,
+    });
+    expect(spy).toHaveBeenCalledTimes(1);
+    rerender({ pl: "5" }); // new object identity, same serialized value
+    expect(spy).toHaveBeenCalledTimes(1);
+  });
+
+  it("clears a key when its value turns null", () => {
+    const { rerender } = renderHook((p: Record<string, string | null>) => useUrlMirror(p), {
+      initialProps: { pl: "5" } as Record<string, string | null>,
+    });
+    expect(window.location.search).toBe("?pl=5");
+    rerender({ pl: null });
+    expect(window.location.search).toBe("");
+    expect(spy).toHaveBeenCalledTimes(2);
+  });
+
+  it("preserves other params already on the URL", () => {
+    setUrl("?page=playlists");
+    renderHook(() => useUrlMirror({ pl: "5" }));
+    expect(window.location.search).toBe("?page=playlists&pl=5");
+  });
+});