feat: allow users to resume previous QA sessions from the discussion panel

## Summary

Allow users to click on a previous QA/Discussion entry in the right-side panel to resume the conversation with full context, rather than losing it once a session ends.

## Motivation

Currently, when a QA or Discussion round ends, users cannot go back to continue the conversation. The context is effectively lost, making it impossible to follow up on earlier points or build on previous discussions.

Ref: https://github.com/THU-MAIC/OpenMAIC/issues/152

## Proposed Approach

- Display completed QA/Discussion sessions as clickable entries in the right-side panel
- Clicking an entry **re-enters the session** with full conversation history, allowing the user to continue asking questions and receiving responses in the same context
- Fallback: if full resumption is not feasible for certain session types, provide at minimum a read-only review view
- Persist QA history across page refreshes (e.g., IndexedDB)

## Area

Classroom playback / Discussion UX