feat: add ThumbHash support alongside BlurHash across events, uploads, and UI

Adds a parallel ThumbHash placeholder everywhere BlurHash is already used.
Remote events now carry both hashes; renderers prefer thumbhash when
available and fall back to blurhash. The MIP-04 `thumbhash` imeta field
that was previously reserved-but-unused is now wired end-to-end.

- New ThumbHash encoder/decoder in commons/commonMain (no new Gradle dep)
- New ThumbhashTag and thumbhash accessors/builders across NIP-17, 68,
  71, 94, 95, 99, the experimental profile gallery, and MIP-04
- RichTextParser + MediaContentModels carry a thumbhash field alongside
  blurhash so every downstream composable can pick its preferred placeholder
- New ThumbHashFetcher (Android + Desktop) registered with Coil, plus a
  small placeholderModel(thumbhash, blurhash) helper centralising the
  "prefer thumbhash, fall back to blurhash" rule
- Rename BlurhashMetadataCalculator -> PreviewMetadataCalculator; the
  calculator decodes the bitmap / video thumbnail once and runs both
  encoders on the same pixels to keep upload cost flat
- DesktopMediaMetadata, MediaUploadResult, and FileHeader now surface
  both hashes through the NIP-96, Blossom, NIP-95, MIP-04, NIP-17 DM,
  Classifieds, picture, video, and long-form upload paths
- Round-trip unit test for the ThumbHash port

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/service/images/DesktopImageLoaderSetup.kt

@@ -60,8 +60,10 @@ object DesktopImageLoaderSetup {
                 add(SkiaGifDecoder.Factory())
                 add(DesktopBase64Fetcher.Factory)
                 add(DesktopBlurHashFetcher.Factory)
+                add(DesktopThumbHashFetcher.Factory)
                 add(DesktopBase64Fetcher.BKeyer)
                 add(DesktopBlurHashFetcher.BKeyer)
+                add(DesktopThumbHashFetcher.TKeyer)
             }.build()
 
     private fun newMemoryCache(): MemoryCache {

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/service/images/DesktopThumbHashFetcher.kt

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.desktop.service.images
+
+import androidx.compose.runtime.Stable
+import coil3.ImageLoader
+import coil3.asImage
+import coil3.decode.DataSource
+import coil3.fetch.FetchResult
+import coil3.fetch.Fetcher
+import coil3.fetch.ImageFetchResult
+import coil3.key.Keyer
+import coil3.request.Options
+import com.vitorpamplona.amethyst.commons.blurhash.toBufferedImage
+import com.vitorpamplona.amethyst.commons.thumbhash.ThumbHashDecoder
+
+data class ThumbhashWrapper(
+    val thumbhash: String,
+)
+
+@Stable
+class DesktopThumbHashFetcher(
+    private val data: ThumbhashWrapper,
+) : Fetcher {
+    override suspend fun fetch(): FetchResult? {
+        val hash = data.thumbhash
+        val platformImage = ThumbHashDecoder.decodeKeepAspectRatio(hash, 25) ?: return null
+        val bufferedImage = platformImage.toBufferedImage()
+        val bitmap = bufferedImageToSkiaBitmap(bufferedImage)
+
+        return ImageFetchResult(
+            image = bitmap.asImage(true),
+            isSampled = false,
+            dataSource = DataSource.MEMORY,
+        )
+    }
+
+    object Factory : Fetcher.Factory<ThumbhashWrapper> {
+        override fun create(
+            data: ThumbhashWrapper,
+            options: Options,
+            imageLoader: ImageLoader,
+        ): Fetcher = DesktopThumbHashFetcher(data)
+    }
+
+    object TKeyer : Keyer<ThumbhashWrapper> {
+        override fun key(
+            data: ThumbhashWrapper,
+            options: Options,
+        ): String = data.thumbhash
+    }
+}
+
+/**
+ * Pick the best Coil model for a media placeholder on Desktop.
+ *
+ * Prefers [ThumbhashWrapper] when a thumbhash is available (better quality, preserves aspect ratio
+ * and alpha) and falls back to [BlurhashWrapper] when only a blurhash is present.
+ */
+fun placeholderModel(
+    thumbhash: String?,
+    blurhash: String?,
+): Any? =
+    when {
+        !thumbhash.isNullOrEmpty() -> ThumbhashWrapper(thumbhash)
+        !blurhash.isNullOrEmpty() -> BlurhashWrapper(blurhash)
+        else -> null
+    }

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/service/upload/DesktopMediaMetadata.kt

@@ -22,6 +22,7 @@ package com.vitorpamplona.amethyst.desktop.service.upload
 
 import com.vitorpamplona.amethyst.commons.blurhash.toBlurhash
 import com.vitorpamplona.amethyst.commons.blurhash.toPlatformImage
+import com.vitorpamplona.amethyst.commons.thumbhash.toThumbhash
 import com.vitorpamplona.quartz.nip01Core.core.toHexKey
 import com.vitorpamplona.quartz.utils.sha256.sha256
 import java.io.File
@@ -34,6 +35,7 @@ data class MediaMetadata(
     val width: Int? = null,
     val height: Int? = null,
     val blurhash: String? = null,
+    val thumbhash: String? = null,
 )
 
 object DesktopMediaMetadata {
@@ -44,6 +46,7 @@ object DesktopMediaMetadata {
         var width: Int? = null
         var height: Int? = null
         var blurhash: String? = null
+        var thumbhash: String? = null
 
         if (mimeType.startsWith("image/")) {
             try {
@@ -51,7 +54,9 @@ object DesktopMediaMetadata {
                 if (image != null) {
                     width = image.width
                     height = image.height
-                    blurhash = image.toPlatformImage().toBlurhash()
+                    val platformImage = image.toPlatformImage()
+                    blurhash = runCatching { platformImage.toBlurhash() }.getOrNull()
+                    thumbhash = runCatching { platformImage.toThumbhash() }.getOrNull()
                 }
             } catch (_: Exception) {
             }
@@ -64,6 +69,7 @@ object DesktopMediaMetadata {
             width = width,
             height = height,
             blurhash = blurhash,
+            thumbhash = thumbhash,
         )
     }
 

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/ComposeNoteDialog.kt

@@ -358,6 +358,7 @@ private fun buildIMetaTags(results: List<UploadResult>): List<IMetaTag> =
             props["dim"] = listOf("${meta.width}x${meta.height}")
         }
         meta.blurhash?.let { props["blurhash"] = listOf(it) }
+        meta.thumbhash?.let { props["thumbhash"] = listOf(it) }
         IMetaTag(url = url, properties = props)
     }
 
@@ -460,6 +461,7 @@ private fun buildPictureMetas(results: List<UploadResult>): List<com.vitorpamplo
                 },
             hash = meta.sha256,
             size = meta.size.toInt(),
+            thumbhash = meta.thumbhash,
         )
     }
 

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/chats/ChatPane.kt

@@ -825,6 +825,7 @@ private suspend fun sendEncryptedFiles(
                         null
                     },
                 blurhash = result.metadata.blurhash,
+                thumbhash = result.metadata.thumbhash,
                 originalHash = result.metadata.sha256,
             )
         account.sendNip17EncryptedFile(template)