Chat correctness pass: attachment-only quotes, honest channel previews, non-occluding attachment viewer

[dmytrotkk]

This pull request improves the handling and display of attachment-only messages in chat, ensuring that messages with only attachments (and no text) are clearly labeled throughout the mobile and web UIs. It also ensures that channel previews update correctly when posts are edited or deleted, and refines the attachment viewer's UI so that controls never obscure the content.

Attachment-only message support and display:

• Added a utility function quotedBodyText (and helper attachmentOnlyLabel) in formatting.ts to generate a user-friendly label (e.g., "Attachment" or "2 attachments") for messages that have no text but do have attachments, ensuring quoted messages and reply banners are never blank or misleading.
• Updated message quoting logic in both the chat screen (chats/[channelId].tsx) and message bubble (message-bubble.tsx) to use quotedBodyText, and passed the new attachment_count field through the relevant data structures and UI components. (apps/mobile/src/app/(tabs)/chats/[channelId].tsxR68, apps/mobile/src/app/(tabs)/chats/[channelId].tsxR663, apps/mobile/src/app/(tabs)/chats/[channelId].tsxL961-R965, [1] [2] [3] [4] [5]

Backend support for attachment counts:

• The backend now computes and returns attachment_count in MmPostParentPreview and its API, so the client can distinguish between truly empty and attachment-only parent messages. [1] [2]

Channel preview consistency:

• When a post is edited or deleted, the denormalized channel preview is recomputed to reflect the latest message, preventing stale or incorrect previews in the chat list. [1] [2]
• The mobile app now invalidates the channel list on post.deleted events to fetch updated previews from the server.

Attachment viewer UI improvements (web):

• Refactored the attachment viewer so that the control bar (chrome) is always outside the media area, never covering the content, and added idle detection to fade controls when the user is just viewing. Navigation chevrons adapt to screen size to avoid shrinking images unnecessarily. [1] [2] [3] [4] [5]