app: make background subagent entries in Session details clickable

Summary

Make the currently running background subagent entries in the OpenCode Web UI's Session details clickable so users can navigate directly to the corresponding child sessions. This request concerns the Web UI, not the TUI.

Environment

• opencode version: opencode2 v0.0.0-beta-19059
• OS: Linux 7.0.0-30-generic (x86_64)
• Terminal: TERM=xterm-256color, TERM_PROGRAM=Orca, COLORTERM=truecolor
• Shell: /bin/bash
• Install/channel: beta
• Active plugins: none configured or found in the global OpenCode configuration

Reproduction

1. Open the OpenCode beta Web UI and open a session.
2. Start a background subagent and wait for it to appear in Session details under “running work in background”.
3. Click the running background subagent entry.
4. Observe that the child session does not open.

Expected Behavior

Clicking a background subagent entry in the “running work in background” section should navigate directly to its child session.

Actual Behavior

The running background subagent entry is non-interactive, so users cannot navigate to the child session from Session details.

Additional Context

The child session ID is already available in task.id. The implementation can reuse the existing sessionHref and navigateToSession patterns used for child-session navigation. No server or API changes should be necessary.