refactor(gallery): use HlsContentTypes constant and tighten comments

Use the existing HlsContentTypes.HLS_PLAYLIST constant for the
canonical HLS playlist mime in GalleryThumb's isHlsMimeType so the
read-side stays in sync with the publish path (HlsVideoEventBuilder,
HlsPublishOrchestrator). Trim two block comments down to the
non-obvious WHY only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/components/ZoomableContentView.kt

@@ -940,10 +940,9 @@ fun ShareMediaAction(
                             if (videoUri != null) {
                                 val n19 = Nip19Parser.uriToRoute(postNostrUri)?.entity as? NEvent
                                 if (n19 != null) {
-                                    // Forward the imeta poster (only MediaUrlVideo carries it) so HLS
-                                    // gallery entries have a still to render — without this, a video
-                                    // event whose imeta only contains a .m3u8 URL renders as a black
-                                    // play-icon tile in the gallery.
+                                    // Forward the imeta poster so HLS gallery entries have a decodable
+                                    // still — the .m3u8 URL itself is a text manifest. Only
+                                    // MediaUrlVideo carries an artworkUri.
                                     val posterUrl = (content as? MediaUrlVideo)?.artworkUri
                                     accountViewModel.addMediaToGallery(n19.hex, videoUri, n19.relay.getOrNull(0), blurhash, dim, hash, mimeType, image = posterUrl)
                                     accountViewModel.toastManager.toast(R.string.media_added, R.string.media_added_to_profile_gallery)

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/profile/gallery/GalleryThumb.kt

@@ -39,6 +39,7 @@ import androidx.media3.common.util.UnstableApi
 import coil3.compose.AsyncImagePainter
 import coil3.compose.SubcomposeAsyncImage
 import coil3.compose.SubcomposeAsyncImageContent
+import com.davotoula.lightcompressor.hls.HlsContentTypes
 import com.vitorpamplona.amethyst.R
 import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
 import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
@@ -66,11 +67,11 @@ import com.vitorpamplona.quartz.nip53LiveActivities.clip.LiveActivitiesClipEvent
 import com.vitorpamplona.quartz.nip68Picture.PictureEvent
 import com.vitorpamplona.quartz.nip71Video.VideoEvent
 
-// HLS playlist mime types as published in NIP-71 imeta tags. Mirrors the canonical list used in
-// MediaItemCache.toExoPlayerMimeType. Kept inline to avoid creating a one-off helper module.
+// Mirrors the canonical HLS-playlist mime list used in MediaItemCache.toExoPlayerMimeType.
+// Kept inline rather than extracting a shared helper for one read-side caller.
 private fun isHlsMimeType(mimeType: String?): Boolean =
     when (mimeType?.lowercase()) {
-        "application/vnd.apple.mpegurl",
+        HlsContentTypes.HLS_PLAYLIST,
         "application/x-mpegurl",
         "audio/x-mpegurl",
         "audio/mpegurl",