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/db.ts

@@ -0,0 +1,32 @@
+import * as SQLite from 'expo-sqlite';
+
+let db: Promise<SQLite.SQLiteDatabase> | null = null;
+
+/**
+ * Локальная база офлайн-режима (docs/mobile.md#офлайн). `owner` — хэш «сервер + пользователь»:
+ * скачанное и неотправленные события одного аккаунта не видны другому.
+ */
+export function getDb(): Promise<SQLite.SQLiteDatabase> {
+  db ??= (async () => {
+    const d = await SQLite.openDatabaseAsync('pipu.db');
+    await d.execAsync(`
+      PRAGMA journal_mode = WAL;
+      CREATE TABLE IF NOT EXISTS downloads (
+        owner    TEXT NOT NULL,
+        track_id TEXT NOT NULL,
+        track    TEXT NOT NULL,
+        status   TEXT NOT NULL,
+        bytes    INTEGER NOT NULL DEFAULT 0,
+        added_at INTEGER NOT NULL,
+        PRIMARY KEY (owner, track_id)
+      );
+      CREATE TABLE IF NOT EXISTS pending_events (
+        id    INTEGER PRIMARY KEY AUTOINCREMENT,
+        owner TEXT NOT NULL,
+        body  TEXT NOT NULL
+      );
+    `);
+    return d;
+  })();
+  return db;
+}