Fix/sliding sync avatars packs

[eleboucher]

Description

Fixes three sliding sync regressions:

• Space icons stayed blank on first load: avatar state arrives after the sidebar renders and nothing re-rendered on it. Space tabs now use the reactive useRoomAvatar hook.
• Bridged users had no timeline avatars: ghosts have no /profile fallback and their m.room.member event isn't in the lazy sync window. The timeline now hydrates missing members on demand, with a 5 min negative cache for failed fetches.
• Packs hosted on space rooms didn't load: the space subscription key won over the image-pack one and doesn't request pack state. Added a space_image_packs composite key, and pack rooms are subscribed at client load instead of on first emoji board open.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).