feat: show last message for ghost group (WPB-25285)

[sbakhtiarov]

https://wearezeta.atlassian.net/browse/WPB-25285

https://wearezeta.atlassian.net/browse/WPB-25285

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What's new in this PR?

Issues

For "ghost" groups (groups pending automatic deletion because they have no
eligible admins), the client should surface the pending-deletion state both in
the conversation list (as the last message) and inside the conversation itself,
and it must not be hidden behind unread-event summaries.

Causes

Kalium now exposes the pending deletion as a conversation-level field
(Conversation.adminlessGroupDeletionTimestamp) instead of a system message, so
the Android side needs to read that field and render the indicator itself.

Solutions

• Conversation list (ConversationMapper): added
  adminlessDeleteAwareLastMessage, which for a non-archived conversation with a
  non-null adminlessGroupDeletionTimestamp overrides the last-message preview
  with the new last_message_adminless_delete_reminder string ("Will be deleted
  soon"), regardless of unread-event counts. Otherwise it falls back to the normal
  toUIPreview.
• Inside the conversation (ConversationScreen / MessageList): surface
  adminlessGroupDeletionTimestamp from ConversationMessagesViewState and render
  an AdminlessGroupDeleteReminderItem — as a standalone item when the list is
  empty, or attached to the newest message (reverse-layout aware) otherwise.
• Refactored SystemMessageItem to extract a reusable SystemMessageContentItem
  so the reminder can be rendered without a backing UIMessage, and pulled the
  reminder content into a standalone adminlessGroupDeleteReminderContent.
• Removed the obsolete system-message plumbing now gone from Kalium:
  UIMessageContent.SystemMessage.AdminlessDeleteReminder and its mapping in
  SystemMessageContentMapper.
• Added string last_message_adminless_delete_reminder → "Will be deleted soon".
• Bumped the Kalium submodule to include the ghost-group support.

Dependencies

Needs releases with:

• Kalium: feat: show last message for ghost group (WPB-25285) kalium#4356 (submodule bump included)