refactor(sidebar): H2 — extract 5 hooks + 5 sub-components (#664)

* refactor(sidebar): H2.1 — extract helpers + constants

* refactor(sidebar): H2.2 — extract useSidebarNewReleasesUnread hook

* refactor(sidebar): H2.3 — extract useSidebarNavDnd hook

* refactor(sidebar): H2.4 — extract 3 hooks (LibraryDropdown + ScrollVisible + PerfProbe)

* refactor(sidebar): H2.5 — extract SidebarPerfProbeModal component

* refactor(sidebar): H2.5–H2.8 — split JSX into per-block components

Sidebar.tsx 938 → 271 LOC. All resulting files under the 400-LOC guideline:
  components/sidebar/SidebarLibraryPicker.tsx       96
  components/sidebar/SidebarActiveJobs.tsx          58
  components/sidebar/SidebarNavBody.tsx            293
  components/sidebar/SidebarPerfProbeModal.tsx     259
  components/sidebar/SidebarPerfProbePhase2.tsx    176

src/utils/sidebarHelpers.ts

@@ -0,0 +1,44 @@
+export const SIDEBAR_NAV_LONG_PRESS_MS = 1000;
+export const SIDEBAR_NAV_LONG_PRESS_MOVE_CANCEL_PX = 10;
+export const SMART_PREFIX = 'psy-smart-';
+export const NEW_RELEASES_UNREAD_STORAGE_PREFIX = 'psy_new_releases_unread_seen_v1';
+export const NEW_RELEASES_UNREAD_SAMPLE_SIZE = 80;
+export const NEW_RELEASES_UNREAD_POLL_MS = 2 * 60 * 1000;
+export const NEW_RELEASES_RESET_DELAY_MS = 5_000;
+/** Max album ids persisted per server/scope; cap must not drop the latest "newest" batch when marking read. */
+export const NEW_RELEASES_SEEN_MAX_IDS = 500;
+
+/** Merge previous seen IDs with the current `getAlbumList(newest)` sample: newest batch is kept in full first, then older seen until `maxIds` (localStorage budget). */
+export function mergeSeenNewReleaseIdsCap(prevSeen: string[], newestBatch: string[], maxIds: number): string[] {
+  const out: string[] = [];
+  const seen = new Set<string>();
+  for (const id of newestBatch) {
+    if (!id || seen.has(id)) continue;
+    seen.add(id);
+    out.push(id);
+  }
+  for (const id of prevSeen) {
+    if (out.length >= maxIds) break;
+    if (!id || seen.has(id)) continue;
+    seen.add(id);
+    out.push(id);
+  }
+  return out;
+}
+
+export function isSmartPlaylistName(name: string): boolean {
+  return (name ?? '').toLowerCase().startsWith(SMART_PREFIX);
+}
+
+export function displayPlaylistName(name: string): string {
+  const n = name ?? '';
+  if (isSmartPlaylistName(n)) return n.slice(SMART_PREFIX.length);
+  return n;
+}
+
+export function isPointerOutsideAsideSidebar(clientX: number, clientY: number): boolean {
+  const aside = document.querySelector('aside.sidebar');
+  if (!aside) return false;
+  const r = aside.getBoundingClientRect();
+  return clientX < r.left || clientX > r.right || clientY < r.top || clientY > r.bottom;
+}