fix: normalize video/x-m4v to video/mp4 when saving to MediaStore

Android's MediaStore rejects video/x-m4v (returned by MimeTypeMap for .m4v
URLs) with IllegalArgumentException. M4V is an MP4 container variant, so
map it to video/mp4 before the ContentResolver.insert call.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/actions/MediaSaverToDisk.kt

@@ -205,7 +205,7 @@ object MediaSaverToDisk {
         contentSource: BufferedSource,
         contentResolver: ContentResolver,
     ) {
-        val cleanMimeType = contentType.substringBefore(";").trim()
+        val cleanMimeType = normalizeMimeTypeForMediaStore(contentType.substringBefore(";").trim())
 
         val (masterUri, baseDir) =
             when {
@@ -271,6 +271,15 @@ object MediaSaverToDisk {
 
     private fun trimInlineMetaData(url: String): String = url.substringBefore("#")
 
+    // Android's MediaStore only accepts a fixed allow-list of MIME types.
+    // MimeTypeMap returns variants like video/x-m4v that MediaProvider rejects,
+    // so map them to the closest supported equivalent.
+    private fun normalizeMimeTypeForMediaStore(mimeType: String): String =
+        when (mimeType.lowercase()) {
+            "video/x-m4v" -> "video/mp4"
+            else -> mimeType
+        }
+
     private const val AMETHYST_SUBDIRECTORY = "Amethyst"
     private const val PDF_MIME_TYPE = "application/pdf"
 }