refactor(settings): co-locate settings feature into features/settings

Theme infra (fixedThemes, theme utils), music-network presetIcon, audio-device
probe, library-index sync, and the auth*SettingsActions slices stay in the
core/global layer (consumed via @/ alias) to avoid inverting core->feature.

src/features/settings/components/backdropReorder.ts

@@ -0,0 +1,36 @@
+import type { BackdropSourcePref } from '@/cover/artistBackdrop';
+
+/**
+ * Move the source at `from` so it lands at exactly index `to` — the ↑/↓ buttons,
+ * i.e. a swap with the neighbour. Returns a new array, or `null` for an
+ * out-of-range or no-op move (so the caller can skip a redundant update).
+ */
+export function moveSourceTo(
+  sources: BackdropSourcePref[],
+  from: number,
+  to: number,
+): BackdropSourcePref[] | null {
+  if (from === to || from < 0 || to < 0 || from >= sources.length || to >= sources.length) return null;
+  const next = sources.slice();
+  const [item] = next.splice(from, 1);
+  next.splice(to, 0, item);
+  return next;
+}
+
+/**
+ * Insert the source at `from` immediately before the row at `beforeIndex` — the
+ * drag-drop case, matching the "line above this row" drop indicator. The -1 when
+ * dragging downward accounts for the slot the dragged item vacated. Returns a
+ * new array, or `null` for an out-of-range or no-op move.
+ */
+export function dropSourceBefore(
+  sources: BackdropSourcePref[],
+  from: number,
+  beforeIndex: number,
+): BackdropSourcePref[] | null {
+  if (from === beforeIndex || from < 0 || from >= sources.length) return null;
+  const next = sources.slice();
+  const [item] = next.splice(from, 1);
+  next.splice(from < beforeIndex ? beforeIndex - 1 : beforeIndex, 0, item);
+  return next;
+}