address review: localize connection errors, tighten i18n guard, drop dead resources

- AddSourceViewModel resolves SubsonicException.uiMessage too, so malformed/
  HTTP/truncated responses show localized text instead of the raw parser
  message; add a regression test.
- StringResourcesTest now requires an 'other' quantity in every plural and
  asserts exact placeholder (index and conversion) equality, both of which
  could previously pass while crashing getQuantityString at runtime.
- Map the pre-enum song_downloads.error sentences back to their DownloadError
  so existing failed rows keep their specific message.
- Localize the media-session star/unstar button label.
- Remove 12 unreferenced resources (queue name, filter year, sync_failed, and
  the unused codec_* strings; codec labels stay in AudioEncoding).
- Make buildTree's fallback arguments required; only tests used the defaults.

app/src/main/res/values-de/strings.xml

@@ -17,7 +17,6 @@
     <string name="resources_filter_artist">Künstler</string>
     <string name="resources_filter_genre">Nach Genre</string>
     <string name="resources_filter_starred">Favoriten</string>
-    <string name="resources_filter_year">Jahr</string>
     <plurals name="resources_song_count">
         <item quantity="one">%1$d Song</item>
         <item quantity="other">%1$d Songs</item>