fix: prune left-elsewhere rooms from sliding sync sidebar cache before hydration

[eleboucher]

Description

With sliding sync, the sidebar briefly showed rooms/spaces you'd left on another device, then removed them once reconciliation ran. This fetches the joined-rooms set before hydration and prunes stale rooms from the cache first, so the sidebar renders once at the correct count. Invites are kept; if the request is slow (2s) or offline, it falls back to the old behaviour.

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).