Refactor current chat state decomposition into focused StateFlows

[fakelog]

Previously, a single broad state object caused unnecessary recompositions of the entire ChatContent screen on unrelated changes (input, search, selection, pinned state, viewers, overlays, appearance flags).

Now state is split by responsibility:

• chatUiState: top bar, screen-level flags, dialogs, topic/navigation state
• messagesState: message list, loading, pagination, scroll commands and viewport state
• selectionState: selected messages and selection mode
• searchState: search query and search mode
• inputState: draft, reply/edit state, bot commands, inline results, sticker sheet
• pinnedState: pinned banner and pinned messages sheet
• mediaViewerState: image/video/web/mini app viewers
• appearanceState: wallpaper and message rendering preferences

Benefits:

• Unrelated updates no longer recompose the whole chat screen
• Heavy UI like the message list subscribes to narrower flows, reducing noise
• Screen state aligns better with actual UI responsibilities, improving maintainability
• Background, message list and message options no longer depend on the monolithic state directly

Additionally, moved ChatContent subtrees to the focused state models so the UI no longer collects the broad component state at the root.