feat: debounced presence transition broadcasts (#218)

[Delightech28]

Description

Resolves #218 by implementing debounced presence transition broadcasts (online/offline).

Key changes:

• Modified the backend presence service to detect aggregate connection changes, avoiding rapid flap events when a user briefly disconnects.
• Debounced offline broadcasts by deferring them for 3 seconds. If the user reconnects within the window, the offline broadcast is canceled.
• Updated the presence_update WebSocket payload to include { userId, status, lastSeen } to provide more accurate state tracking to peers.
• Updated the frontend ConversationListSidebar component to gracefully handle the new status payload in addition to the legacy boolean structure.

Type of change

• Bug fix
• New feature
• Documentation update
• Other

Checklist

• I have read the contributing guidelines
• I have tested my changes locally
• My code follows the project's coding standards

closes #218

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[codebestia]

@Delightech28 Please fix the CI