Feature: Mention-priority unread notifications (@username)

[mjkatgithub]

Goal

Prioritize mention notifications so channels with direct mentions are highlighted
more strongly than normal unread channels.

Scope

• Detect direct mentions of current user (@username) in channel messages.
• Distinguish mention-unread state from normal unread state.
• Render mention-unread indicator in red for stronger visibility.
• Keep mention indicator in sync when mention is read/acknowledged.
• Preserve normal unread behavior for non-mention messages.

Branch

feature/mention-priority-notifications

Acceptance Criteria

• Channels with @username mentions are highlighted in red.
• Red mention indicator has higher visual priority than normal unread state.
• Reading the mentioned message clears mention indicator correctly.
• Non-mention unread channels keep standard unread styling.
• Mention detection and indicator updates work in real-time.

Test Checklist

Unit

• Mention detection for current username in message content/events.
• Priority resolution between mention-unread and normal unread states.

Integration

• Channel list applies red indicator for mention-unread channels.
• Read/acknowledge flow clears mention state correctly.

E2E

• Receive @username in non-active channel and verify red indicator.
• Receive normal message in another channel and verify standard indicator.
• Open mentioned channel and verify red indicator is cleared.

Notes

• Ensure contrast/accessibility for red indicator in dark and light themes.
• Mention notification behavior should not break existing unread logic.