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/test/kotlin/com/subtracks/i18n/StringResourcesTest.kt

@@ -24,6 +24,12 @@ class StringResourcesTest {
         val default = parse(defaultFile)
         assertTrue("default strings.xml should not be empty", default.isNotEmpty())
 
+        default.forEach { (name, resource) ->
+            if (resource.kind == "plurals") {
+                assertTrue("$name in values/strings.xml is missing the required 'other' quantity", "other" in resource.quantities)
+            }
+        }
+
         localeFiles().forEach { file ->
             val locale = parse(file)
             locale.forEach { (name, resource) ->
@@ -35,12 +41,12 @@ class StringResourcesTest {
                         "$name has an invalid quantity in ${file.parentFile.name}",
                         resource.quantities.all { it in validQuantities },
                     )
+                    assertTrue("$name in ${file.parentFile.name} is missing the required 'other' quantity", "other" in resource.quantities)
                 }
-                val baseArgs = placeholderIndices(base.text)
-                val localeArgs = placeholderIndices(resource.text)
-                assertTrue(
-                    "$name in ${file.parentFile.name} uses placeholders $localeArgs not present in the default $baseArgs",
-                    baseArgs.containsAll(localeArgs),
+                assertEquals(
+                    "$name placeholders differ in ${file.parentFile.name}",
+                    placeholders(base.text),
+                    placeholders(resource.text),
                 )
             }
         }
@@ -84,6 +90,6 @@ class StringResourcesTest {
         return (0 until items.length).joinToString(" ") { items.item(it).textContent }
     }
 
-    private fun placeholderIndices(text: String): Set<Int> =
-        Regex("""%(\d+)\$""").findAll(text).mapTo(mutableSetOf()) { it.groupValues[1].toInt() }
+    private fun placeholders(text: String): Map<Int, Char> =
+        Regex("""%(\d+)\$([a-zA-Z])""").findAll(text).associate { it.groupValues[1].toInt() to it.groupValues[2][0] }
 }

app/src/test/kotlin/com/subtracks/ui/downloads/DownloadTreeTest.kt

@@ -17,6 +17,8 @@ class DownloadTreeTest {
                     song("s3", "Three", "ar1", "Artist", "al2", "Second", size = 5),
                     song("s4", "Four", "ar2", "Other", "al3", "Third", size = 1),
                 ),
+                "Unknown artist",
+                "Unknown album",
             )
 
         assertEquals(listOf("Artist", "Other"), tree.artists.map { it.name })
@@ -37,7 +39,7 @@ class DownloadTreeTest {
 
     @Test
     fun aSongWithNoArtistOrAlbumIsStillShown() {
-        val tree = buildTree(listOf(song("s1", "One", null, null, null, null, size = 3)))
+        val tree = buildTree(listOf(song("s1", "One", null, null, null, null, size = 3)), "Unknown artist", "Unknown album")
 
         val artist = tree.artists.single()
         assertEquals("Unknown artist", artist.name)

app/src/test/kotlin/com/subtracks/ui/settings/AddSourceViewModelTest.kt

@@ -104,6 +104,20 @@ class AddSourceViewModelTest {
         assertEquals("Failed: Wrong username", state.message)
     }
 
+    @Test
+    fun aMalformedResponseIsReportedWithTheLocalizedMessage() {
+        val viewModel = viewModel()
+        viewModel.setAddress(server.url("/").toString())
+        viewModel.setUsername("u")
+        viewModel.setPassword("p")
+        server.enqueue(MockResponse().setBody("not an xml document"))
+
+        viewModel.testConnection()
+        await { !viewModel.state.value.busy && viewModel.state.value.isError }
+
+        assertEquals("Malformed response from the server", viewModel.state.value.message)
+    }
+
     @Test
     fun aSuccessfulPingClearsAnEarlierError() {
         val viewModel = viewModel()