feat(sidebar): long-press drag to reorder and hide nav items (#269)

Hold for 1s then drag to reorder configurable library and system links;
drop outside the sidebar to hide items (same visibility as Settings).
Show a trash hint near the cursor when outside removal applies.
Extract shared reorder helpers into src/utils/sidebarNavReorder.ts and
reuse them from the Settings sidebar customizer.

src/utils/sidebarNavReorder.ts

@@ -0,0 +1,67 @@
+import { ALL_NAV_ITEMS } from '../config/navItems';
+import type { SidebarItemConfig } from '../store/sidebarStore';
+
+export type SidebarNavSection = 'library' | 'system';
+
+export type SidebarNavDropTarget = {
+  idx: number;
+  before: boolean;
+  section: SidebarNavSection;
+};
+
+export function getLibraryItemsForReorder(
+  items: SidebarItemConfig[],
+  randomNavMode: 'hub' | 'separate',
+): SidebarItemConfig[] {
+  return items.filter(cfg => {
+    if (!ALL_NAV_ITEMS[cfg.id] || ALL_NAV_ITEMS[cfg.id].section !== 'library') return false;
+    if (randomNavMode === 'hub' && (cfg.id === 'randomMix' || cfg.id === 'randomAlbums')) return false;
+    if (randomNavMode === 'separate' && cfg.id === 'randomPicker') return false;
+    return true;
+  });
+}
+
+export function getSystemItemsForReorder(items: SidebarItemConfig[]): SidebarItemConfig[] {
+  return items.filter(cfg => ALL_NAV_ITEMS[cfg.id]?.section === 'system');
+}
+
+/** Same entries as in Settings toggles — safe to hide via drag-out. */
+export function isSidebarNavItemUserHideable(id: string): boolean {
+  return Boolean(ALL_NAV_ITEMS[id]);
+}
+
+/**
+ * Reorders one sidebar section (library or system) like the Settings customizer.
+ * Returns a new `items` array, or null if nothing changes.
+ */
+export function applySidebarDropReorder(
+  allItems: SidebarItemConfig[],
+  section: SidebarNavSection,
+  fromIdx: number,
+  target: SidebarNavDropTarget | null,
+  randomNavMode: 'hub' | 'separate',
+): SidebarItemConfig[] | null {
+  if (!target || target.section !== section) return null;
+
+  const sectionItems =
+    section === 'library'
+      ? [...getLibraryItemsForReorder(allItems, randomNavMode)]
+      : [...getSystemItemsForReorder(allItems)];
+
+  const insertBefore = target.before ? target.idx : target.idx + 1;
+  if (insertBefore === fromIdx || insertBefore === fromIdx + 1) return null;
+
+  const [moved] = sectionItems.splice(fromIdx, 1);
+  sectionItems.splice(insertBefore > fromIdx ? insertBefore - 1 : insertBefore, 0, moved);
+
+  const visibleIds = new Set(sectionItems.map(c => c.id));
+  const next = [...allItems];
+  const positions = next
+    .map((cfg, i) => ({ cfg, i }))
+    .filter(({ cfg }) => visibleIds.has(cfg.id))
+    .map(({ i }) => i);
+  positions.forEach((pos, i) => {
+    next[pos] = sectionItems[i];
+  });
+  return next;
+}