refactor(atoms): convert atoms to presentational components

[raaymax]

Summary

Refactors atom components to follow the presentational component pattern. Atoms now receive all data via props instead of accessing global state or API clients directly, improving testability and separation of concerns.

Changes

Atoms (now pure presentational)

• Image: Remove MobX observer and client dependency; receive src, downloadUrl, size via props
• File: Remove observer wrapper; receive file data via props
• ProfilePic: Remove observer and useApp hook; receive avatarUrl and status via props
• ThemeButton: Simplify to receive theme and onToggle via props

Moved to Molecules

• LoggedUser: Moved from atoms to molecules since it requires app context for user data
• ThemeButtonConnected: New molecule that wraps ThemeButton with theme context

Updated Parent Components

• Files: Now resolves file URLs before passing to Image/File atoms
• Message: Passes resolved avatar URLs to ProfilePic
• NavUsers: Passes user data props to ProfilePic
• ReadReceipt: Passes avatar URLs to ProfilePic
• ThreadInfo: Passes resolved props to child atoms
• DiscussionHeader: Updated to use new component APIs
• Sidebar: Uses ThemeButtonConnected instead of ThemeButton

Testing

• Storybook stories updated for all changed atoms
• Stories now provide mock data directly instead of relying on global state

Notes

This follows atomic design principles where atoms should be stateless and receive all data via props, making them easier to test and reuse.