Display number of unread messages in room list

[sgrimee]

Summary

Add visual indicators in the room list showing the count of unread messages for each room/conversation.

Requirements

• Display unread message count next to room names
• Update counts in real-time as messages arrive
• Clear counts when room is viewed/messages are read
• Handle different types of unread indicators (numbers, badges, etc.)

UI Considerations

• Compact display format for room list
• Different styling for rooms with/without unread messages
• Consider maximum count display (e.g., "99+" for high counts)
• Integration with existing room list filtering

Dependencies

• Related to issue Highlight unread messages in room view #79 (Highlight unread messages in room view)
• Should coordinate with message read/unread tracking
• Consider future theming system compatibility (issue Add configurable theming system #80)

Implementation Notes

• Extend existing room list UI components
• Track unread counts in room cache/state
• Update counts from message events and room viewing
• Consider persistence of unread counts between sessions

Acceptance Criteria

• Unread message counts appear in room list
• Counts update automatically when messages arrive
• Counts clear when rooms are viewed
• UI remains clean and readable with count indicators
• Performance acceptable with many rooms and high message volume

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