feat(android): home-screen download queue widget

A RemoteViews widget (no new dependencies) shows the active track,
artist, queue count, and a coarse progress bar, with tap-to-open.
DownloadService pushes updates only on discrete events — track or
status changes and 25% progress steps, never per byte — matching the
battery discipline; the last state persists so launcher refreshes
render after process death. Colors use Material You system tokens on
API 31+ with static light/dark fallbacks below.

android/app/src/main/AndroidManifest.xml

@@ -118,6 +118,17 @@
             android:exported="false"
             android:foregroundServiceType="dataSync" />
 
+        <receiver
+            android:name=".DownloadQueueWidgetProvider"
+            android:exported="false">
+            <intent-filter>
+                <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
+            </intent-filter>
+            <meta-data
+                android:name="android.appwidget.provider"
+                android:resource="@xml/widget_download_queue_info" />
+        </receiver>
+
         <service
             android:name="com.ryanheise.audioservice.AudioService"
             android:foregroundServiceType="mediaPlayback"