fix: use the Topic icon for the composer's subject toggle

The subject/title toggle used the Article (document-lines) glyph, which reads as
"body text". Switch to Topic — a clearer signifier for a subject/title line.
Codepoint already present in MaterialSymbols, so no font-subset regen needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B5MLY4hq5LXJ2D5WeLRyXj

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

@@ -916,7 +916,7 @@ private fun AddSubjectButton(
 ) {
     IconButton(onClick = onClick) {
         Icon(
-            symbol = MaterialSymbols.AutoMirrored.Article,
+            symbol = MaterialSymbols.Topic,
             contentDescription = stringRes(R.string.messages_new_message_subject),
             modifier = Modifier.height(22.dp),
             tint = if (isActive) MaterialTheme.colorScheme.primary else MaterialTheme.colorScheme.onBackground,