feat(mobile): offline downloads, persistent event queue and offline start (M5)

- Downloads queue (2 parallel, Wi-Fi only by default) stores files under
  the app's document directory and track metadata in SQLite; Collection
  gets a Downloads tab, album and player screens get download buttons.
- Player plays downloaded files and local covers; My Wave falls back to
  a shuffle of downloaded tracks when the server is unreachable.
- Playback events that fail on the network are kept in SQLite and sent
  when the connection returns.
- AuthSession can open offline with the cached user and fetches an
  access token before the first request once online.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/mobile/src/offline/events.ts

@@ -0,0 +1,72 @@
+import { ApiError, unwrap, type AuthSession, type PlayEvent } from '@pipu/core';
+
+import { shortHash } from '../lib/storage';
+import { getDb } from './db';
+
+const BATCH = 500;
+
+/**
+ * Отправка событий прослушивания с запасом на офлайн: что не ушло из-за сети — в SQLite,
+ * досылается при появлении сети (`POST /events` идемпотентен по id события).
+ */
+export class PersistentEvents {
+  private readonly owner: string;
+  private draining = false;
+
+  constructor(
+    private readonly session: AuthSession,
+    server: string,
+    userId: string,
+  ) {
+    this.owner = shortHash(`${server}|${userId}`);
+  }
+
+  private async post(events: PlayEvent[]) {
+    unwrap(await this.session.client.POST('/api/v1/events', { body: { events } }));
+  }
+
+  /** Sink для EventBuffer: при сетевой ошибке сохраняет пачку и «успешно» завершается. */
+  send = async (events: PlayEvent[]) => {
+    try {
+      await this.post(events);
+      void this.drain();
+    } catch (e) {
+      if (!isNetwork(e)) throw e;
+      const db = await getDb();
+      await db.runAsync('INSERT INTO pending_events (owner, body) VALUES (?, ?)', this.owner, JSON.stringify(events));
+    }
+  };
+
+  /** Досылает сохранённые пачки; останавливается на первой сетевой ошибке. */
+  async drain() {
+    if (this.draining) return;
+    this.draining = true;
+    try {
+      const db = await getDb();
+      for (;;) {
+        const rows = await db.getAllAsync<{ id: number; body: string }>(
+          'SELECT id, body FROM pending_events WHERE owner = ? ORDER BY id LIMIT 20',
+          this.owner,
+        );
+        if (!rows.length) return;
+        for (const r of rows) {
+          const events = JSON.parse(r.body) as PlayEvent[];
+          try {
+            for (let i = 0; i < events.length; i += BATCH) await this.post(events.slice(i, i + BATCH));
+          } catch (e) {
+            if (isNetwork(e)) return;
+            // Сервер отверг пачку (4xx) — повтор не поможет, отбрасываем.
+          }
+          await db.runAsync('DELETE FROM pending_events WHERE id = ?', r.id);
+        }
+      }
+    } finally {
+      this.draining = false;
+    }
+  }
+}
+
+function isNetwork(e: unknown): boolean {
+  if (e instanceof ApiError) return e.code === 'common.network' || e.status >= 500;
+  return true; // fetch бросает TypeError без ответа сервера
+}