feat(web/m7-364): localStorage persistence layer for queue

web/src/lib/player/persisted.ts

@@ -0,0 +1,55 @@
+import type { TrackRef } from '$lib/api/types';
+
+export type PersistedPlayerState = {
+  v: 1;
+  queue: TrackRef[];
+  index: number;
+  position: number;
+  savedAt: number;
+};
+
+const persistKey = (userId: string) => `minstrel.queue.${userId}`;
+
+export function readPersistedQueue(userId: string): PersistedPlayerState | null {
+  if (typeof localStorage === 'undefined') return null;
+  try {
+    const raw = localStorage.getItem(persistKey(userId));
+    if (!raw) return null;
+    const parsed = JSON.parse(raw) as Partial<PersistedPlayerState>;
+    if (parsed.v !== 1) return null;
+    if (!Array.isArray(parsed.queue)) return null;
+    if (typeof parsed.index !== 'number') return null;
+    if (typeof parsed.position !== 'number') return null;
+    return parsed as PersistedPlayerState;
+  } catch {
+    return null;
+  }
+}
+
+export function writePersistedQueue(
+  userId: string,
+  state: { queue: TrackRef[]; index: number; position: number }
+): void {
+  if (typeof localStorage === 'undefined') return;
+  try {
+    const payload: PersistedPlayerState = {
+      v: 1,
+      queue: state.queue,
+      index: state.index,
+      position: state.position,
+      savedAt: Date.now()
+    };
+    localStorage.setItem(persistKey(userId), JSON.stringify(payload));
+  } catch {
+    // QuotaExceededError, security errors, etc. Swallow; persistence is best-effort.
+  }
+}
+
+export function clearPersistedQueue(userId: string): void {
+  if (typeof localStorage === 'undefined') return;
+  try {
+    localStorage.removeItem(persistKey(userId));
+  } catch {
+    /* ignore */
+  }
+}