Unread/pending activity indicator on session items

[kwad-pylot]

Description

Show a visual indicator (badge, dot, or highlight) on session items in the left sidebar when there is activity requiring the user's attention — such as Claude finishing a response, a permission request waiting, or new messages arriving while the user is in a different session.

Acceptance Criteria

• Visual indicator (dot/badge) on session items with pending activity
• Triggers: Claude finished responding, permission prompt waiting, new messages
• Indicator clears when user opens that session
• Distinguishable from the existing green "active" pulse

[viper151]

@blackmammoth this could be considered for the backend refactoring perhaps as a use case.