Add a pending-answer thread badge for active questions #877

Problem

Threads currently show badges like Completed and Working, but there is no distinct badge for the state where a question is active and the thread is waiting on an answer.

Desired behavior

Add a thread badge for pending answers so users can quickly identify threads with an active question awaiting resolution.

This badge should be visually aligned with the existing Completed and Working badges and fit into the same thread-status system.

Why this matters

A thread that is actively awaiting an answer is meaningfully different from one that is currently working or already completed. Without a dedicated badge, it is harder to scan the thread list and understand which conversations still need attention.

Acceptance criteria

• Threads can display a dedicated pending-answer badge when a question is active
• The badge appears alongside the existing status badges pattern used for Completed and Working
• The badge state is driven by thread/session state rather than manual user input
• Users can distinguish at a glance between Working, Completed, and Pending answer states