feat(app): back button in subagent sessions

[tomzhu1024]

What does this PR do?

Added a back button alongside the title in subagent sessions, allowing users to return to the parent session.

Closes #10219

How did you verify your code works?

I ran the web locally and checked this feature.

Screen.Recording.2026-01-24.at.3.46.30.PM.mov

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, the most relevant related PRs are:

1. feat(task): Add subagent-to-subagent delegation with budgets, persistent sessions, and hierarchical session navigation (PR feat(task): Add subagent-to-subagent delegation with budgets, persistent sessions, and hierarchical session navigation #7756)

   • Related because it handles hierarchical session navigation and parent-child session relationships

2. feat: add subagents sidebar with clickable navigation and parent keybind (PR feat: add subagents sidebar with clickable navigation and parent keybind #4865)

   • Related because it provides navigation features for subagents and parent session access

3. feat: add interactive session switcher for subagent navigation (PR feat: add interactive session switcher for subagent navigation #6184)

   • Related because it addresses subagent navigation, which overlaps with back button functionality

4. Desktop: Sidebar subsessions support (PR Desktop: Sidebar subsessions support #6368)

   • Related because it provides sidebar support for subsessions

These PRs all deal with subagent navigation and session hierarchy. However, none appear to be direct duplicates of the current PR #10439, which specifically implements a back button to return to the parent session. These are related features in the broader subagent navigation space but address different aspects.