feat(client): direct message UI on Android (beta blocker 4/5)

[Detair]

Summary

Android beta blocker 4/5: direct messages are now reachable from the Android client.

The home screen gains an envelope action (Email icon, core material-icons set) that opens a conversation list showing each DM's avatar, title, last-message preview, and unread badge. Tapping a conversation opens it through the existing TextChannelScreen — a DM is a channel, so its messages already flow through the message API by channel id.

What's new

• Models — DmConversation / DmParticipant / LastMessagePreview matching the flattened GET /api/dm response. KaikuJson uses the SnakeCase naming strategy + ignoreUnknownKeys, so the unmodelled channel fields (guild_id, position, …) parse away cleanly.
• DmApi (getDms + markAsRead) and DmModule @Binds, following the existing MessageApi/ChatModule pattern.
• DmRepository (@Singleton) holding the conversation list, with pure, unit-tested companion helpers:
  • conversationTitle — other participant for 1:1, group name (or joined names) for group DMs
  • conversationAvatarUrl — other participant's avatar for 1:1, group icon otherwise
  • clearUnread
• DmListScreen + DmListViewModel — loads on open, retry on error, empty state; opening a DM marks it read (optimistic local clear + server call).
• Navigation — new composable("dms") route; entry point threaded as onNavigateToDms through HomeScreen (defaulted param, so no other caller breaks).

Title/avatar resolution

Conversation	Title	Avatar
1:1 DM	other participant's display name	other participant's avatar
group w/ name	channel name	group icon
group w/o name	other participants joined by ,	group icon

Tests

DmConversationTest covers all title/avatar branches (1:1, group named, group unnamed, self-only fallback) and unread clearing — 9 cases.

Notes / known limitations (acceptable for beta)

• The DM list refreshes on open; it does not yet update unread live from the WebSocket while the list is on screen.
• Avatar URLs are passed to Coil as-is, matching the existing MessageItem avatar convention.

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