net: send the subtracks user agent on every request

Only SubsonicClient tagged requests with 'subtracks/android'; playback (Media3 OkHttpDataSource), downloads (DownloadManager) and artwork (Coil/OkHttpArtworkFetcher) fell back to the OkHttp or platform default. Add a single UserAgent with the app version, apply it to the shared OkHttp client, the streaming data source factory and the DownloadManager request, and drop SubsonicClient's now-redundant header.

app/src/test/kotlin/com/subtracks/UserAgentTest.kt

@@ -0,0 +1,24 @@
+package com.subtracks
+
+import okhttp3.OkHttpClient
+import okhttp3.Request
+import okhttp3.mockwebserver.MockResponse
+import okhttp3.mockwebserver.MockWebServer
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class UserAgentTest {
+    @Test
+    fun addsTheSubtracksUserAgent() {
+        val server = MockWebServer()
+        server.enqueue(MockResponse())
+        server.start()
+        val client = OkHttpClient.Builder().addInterceptor(UserAgent.interceptor()).build()
+
+        client.newCall(Request.Builder().url(server.url("/")).build()).execute().close()
+        val header = server.takeRequest().getHeader("User-Agent")
+        server.shutdown()
+
+        assertEquals(UserAgent.value, header)
+    }
+}