Backport from the iOS version: offline downloads, home-first reconnect, stall warning, Close

- Downloads for offline listening: long-press a track, album, artist or
  playlist -> Download; Menu -> Downloads lists them (progress, space used/free,
  play/shuffle, retry, remove). Saved on the SD card (Android/data/com.minamp/
  files/downloads). MP3 originals kept as they are unless above a lower
  download quality; the rest transcoded to MP3 (320 kbps by default). One at a
  time, Range resume for originals, retried when the server is back, Wi-Fi lock.
- Downloaded tracks play from the SD card, offline too; plays made offline
  are reported to Plex when a connection works again.
- Server.pick: when last time's connection was remote, give the plain home
  addresses 2 s first (a hairpinned remote address kept us encrypted and
  capped at home). test/PreferHomeTest.java.
- Playback "playing" but not moving for 12 s: say so instead of staying silent.
- Menu -> Close Minamp: stop the music and close every screen.

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

test/identity_server.py

@@ -0,0 +1,15 @@
+"""Answers /identity like a Plex Media Server (for PreferHomeTest). Usage: identity_server.py PORT"""
+import json, sys
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+
+class H(BaseHTTPRequestHandler):
+    def log_message(self, *a): pass
+    def do_GET(self):
+        body = json.dumps({"MediaContainer": {"machineIdentifier": "abc123"}}).encode()
+        self.send_response(200)
+        self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(body)))
+        self.end_headers()
+        self.wfile.write(body)
+
+ThreadingHTTPServer(("127.0.0.1", int(sys.argv[1])), H).serve_forever()