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/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;
+}