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();
+    });
   });
 });