Fix/sliding sync room state and unread - #1597

Description

• Per-client conn_id: generate a unique id per client instead of a shared constant.
• Server support gate: check /versions before enabling sliding sync; fall back to classic sync if unsupported instead of retrying forever.
• Unscoped receipts: stop scoping the receipts extension so unread state works for every sidebar room.
• List coverage: remove the shrinking steady-state window and unused updates list; extend ranges to cover newly joined rooms instead.
• Leave cleanup: extract removePassiveSubscriptions to also clear image-pack and deferred subscriptions on leave.
• Response safety: wrap response corrections in try/catch so a throw can't wedge the sync loop.

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).