fix: preserve user-provided media descriptions for accessibility

The blanket NIP-31 alt removal also dropped genuine accessibility
descriptions (image descriptions for the blind) on a few media paths where
the user's caption was only stored in the event-level alt tag. Restore them
via the proper, non-deprecated fields:

- NIP-94 FileHeaderEvent (kind 1063) and NIP-17 encrypted file headers
  (kind 15): write the `alt` tag only when the user actually provided a
  caption (the NIP-94 accessibility description), never the old boilerplate
  fallback.
- MIP-04 encrypted group media (kind 9): route the caption into the imeta
  `alt` field via buildMip04IMetaTag instead of an event-level alt tag.

Re-adds the narrowly-scoped TagArrayBuilder.alt() / AltTag.assemble() write
helpers (documented as accessibility-only, not for deprecated NIP-31
boilerplate). Boilerplate alt tags on all other event kinds remain removed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014xAESAz1H1VNjmQpMVqBXj

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/AccountViewModel.kt

@@ -1747,7 +1747,6 @@ class AccountViewModel(
         nostrGroupId: String,
         url: String,
         imeta: com.vitorpamplona.quartz.nip92IMeta.IMetaTag,
-        caption: String? = null,
     ) {
         val template =
             eventTemplate(

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/marmotGroup/send/MarmotFileSender.kt

@@ -44,13 +44,13 @@ class MarmotFileSender(
                     dimensions = upload.dimensions,
                     blurhash = upload.blurhash,
                     thumbhash = upload.thumbhash,
+                    alt = upload.caption,
                 )
 
             accountViewModel.sendMarmotGroupMediaMessage(
                 nostrGroupId = nostrGroupId,
                 url = upload.url,
                 imeta = imeta,
-                caption = upload.caption,
             )
         }
     }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/privateDM/send/upload/ChatFileSender.kt

@@ -28,6 +28,7 @@ import com.vitorpamplona.quartz.nip01Core.tags.references.references
 import com.vitorpamplona.quartz.nip17Dm.base.ChatroomKey
 import com.vitorpamplona.quartz.nip17Dm.files.ChatMessageEncryptedFileHeaderEvent
 import com.vitorpamplona.quartz.nip17Dm.messages.ChatMessageEvent
+import com.vitorpamplona.quartz.nip31Alts.alt
 import com.vitorpamplona.quartz.nip36SensitiveContent.contentWarning
 import com.vitorpamplona.quartz.nip92IMeta.imetas
 import com.vitorpamplona.quartz.utils.ciphers.AESGCM
@@ -65,6 +66,11 @@ class ChatFileSender(
                 blurhash = result.fileHeader.blurHash?.blurhash,
                 thumbhash = result.fileHeader.thumbHash?.thumbhash,
             ) {
+                // NIP-94 accessibility description of the file (kept; the deprecated
+                // generic NIP-31 boilerplate alt is not written).
+                if (!caption.isNullOrEmpty()) {
+                    alt(caption)
+                }
                 contentWarningReason?.let { contentWarning(it) }
             },
         )

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mip04EncryptedMedia/Mip04IMetaTag.kt

@@ -22,6 +22,7 @@ package com.vitorpamplona.quartz.marmot.mip04EncryptedMedia
 
 import com.vitorpamplona.quartz.nip01Core.core.hexToByteArray
 import com.vitorpamplona.quartz.nip01Core.core.toHexKey
+import com.vitorpamplona.quartz.nip31Alts.AltTag
 import com.vitorpamplona.quartz.nip92IMeta.IMetaTag
 import com.vitorpamplona.quartz.nip92IMeta.IMetaTagBuilder
 import com.vitorpamplona.quartz.utils.Log
@@ -167,6 +168,7 @@ fun buildMip04IMetaTag(
     dimensions: String? = null,
     blurhash: String? = null,
     thumbhash: String? = null,
+    alt: String? = null,
 ): IMetaTag =
     IMetaTagBuilder(url)
         .apply {
@@ -178,4 +180,6 @@ fun buildMip04IMetaTag(
             dimensions?.let { add(Mip04Fields.DIMENSIONS, it) }
             blurhash?.let { add(Mip04Fields.BLURHASH, it) }
             thumbhash?.let { add(Mip04Fields.THUMBHASH, it) }
+            // imeta accessibility description (NIP-92/94 "alt"); only when provided.
+            alt?.ifBlank { null }?.let { add(AltTag.TAG_NAME, it) }
         }.build()

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip31Alts/AltTag.kt

@@ -35,5 +35,7 @@ class AltTag {
             ensure(tag[1].isNotEmpty()) { return null }
             return tag[1]
         }
+
+        fun assemble(altDescriptor: String) = arrayOf(TAG_NAME, altDescriptor)
     }
 }

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip31Alts/TagArrayBuilderExt.kt

@@ -0,0 +1,33 @@
+/*
+ * 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.quartz.nip31Alts
+
+import com.vitorpamplona.quartz.nip01Core.core.Event
+import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
+
+/**
+ * Writes an `alt` tag. The generic NIP-31 "alt" client-hint is deprecated, so do
+ * NOT use this to add a boilerplate description of the event kind. It remains only
+ * for file-metadata events (e.g. NIP-94 kind 1063, NIP-17 encrypted file headers)
+ * where the `alt` tag is the accessibility description of the file itself, and only
+ * when the user actually provided one.
+ */
+fun <T : Event> TagArrayBuilder<T>.alt(altDescriptor: String) = addUnique(AltTag.assemble(altDescriptor))

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip94FileMetadata/FileHeaderEvent.kt

@@ -26,6 +26,7 @@ import com.vitorpamplona.quartz.nip01Core.core.HexKey
 import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
 import com.vitorpamplona.quartz.nip01Core.core.any
 import com.vitorpamplona.quartz.nip01Core.signers.eventTemplate
+import com.vitorpamplona.quartz.nip31Alts.alt
 import com.vitorpamplona.quartz.nip50Search.SearchableEvent
 import com.vitorpamplona.quartz.nip94FileMetadata.tags.BlurhashTag
 import com.vitorpamplona.quartz.nip94FileMetadata.tags.DimensionTag
@@ -99,6 +100,9 @@ class FileHeaderEvent(
             initializer: TagArrayBuilder<FileHeaderEvent>.() -> Unit = {},
         ) = eventTemplate(KIND, caption ?: "", createdAt) {
             url(url)
+            // NIP-94 accessibility description of the file (kept; the deprecated
+            // generic NIP-31 boilerplate alt is not written).
+            caption?.ifBlank { null }?.let { alt(it) }
             initializer()
         }
 
@@ -118,6 +122,9 @@ class FileHeaderEvent(
             initializer: TagArrayBuilder<FileHeaderEvent>.() -> Unit = {},
         ) = eventTemplate(KIND, caption ?: "", createdAt) {
             url(url)
+            // NIP-94 accessibility description of the file (kept; the deprecated
+            // generic NIP-31 boilerplate alt is not written).
+            caption?.ifBlank { null }?.let { alt(it) }
 
             hash?.let { hash(it) }
             size?.let { fileSize(it) }