refactor(chats): drop redundant !! on chat-message edit callback

The `if (canEditBuzz || canEditConcord)` guard already proves
`onWantsToEditChatMessage` non-null — both booleans are local vals whose
definitions begin with a null check, and K2 propagates that through them.
The `!!` compiled to an assertion that could never fire, and produced an
"Unnecessary non-null assertion" compiler warning.

No behaviour change: the smart-cast invoke is what was already happening.
If either guard is later loosened, this now fails at compile time instead
of becoming a runtime NPE.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WopdqNoZ9tYoMNppJG17BL

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/feed/ChatMessageActionSheet.kt

@@ -287,7 +287,7 @@ fun ChatMessageActionSheet(
                 TileRow {
                     val label = if (canEditBuzz) R.string.buzz_edit_message else R.string.edit_message
                     ActionTile(MaterialSymbols.Edit, stringRes(label)) {
-                        onWantsToEditChatMessage!!(note)
+                        onWantsToEditChatMessage(note)
                         onDismiss()
                     }
                 }