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

commons/src/androidMain/kotlin/com/vitorpamplona/amethyst/commons/thumbhash/BitmapUtils.kt

@@ -0,0 +1,26 @@
+/*
+ * 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.commons.thumbhash
+
+import android.graphics.Bitmap
+import com.vitorpamplona.amethyst.commons.blurhash.toPlatformImage
+
+fun Bitmap.toThumbhash(): String = this.toPlatformImage().toThumbhash()