Previously, ChatContent and chat viewers collected a single broad chat state, which caused the whole current chat screen to recompose on unrelated changes (messages, wallpaper, selection, pinned state, overlays, input, media viewers).

Now state is split by responsibility:
- chatUiState: chat chrome, metadata, topics, moderation, counters
- messagesState: message list, loading, scroll commands, unread separator
- appearanceState: typography, wallpaper, playback and download preferences
- inputState: draft, reply/editing, mentions, inline bot and attach menu state
- pinnedState: pinned message and pinned list state
- mediaViewerState: image/video/web/mini app overlays
- selectionState/searchState: message selection and in-chat search

Benefits:
- ChatContent subscribes only to the state slices each section actually uses
- Media viewers and menus avoid recomposing on unrelated message or input updates
- MainContent can render overlays without collecting the full chat state
- Current chat state model now matches screen responsibilities and is easier to maintain

Additionally, added a local auto-scroll guard in ChatContent to scope message-count driven scrolling to the active chat/topic and reduce redundant scroll triggers.