## Summary

This PR adds the UI for displaying subagent tool calls:

### Thread view changes
- Add `expanded_subagents` state HashMap for tracking expanded cards  
- Implement `render_subagent_tool_call()` for collapsed card with label
and chevron
- Add subagent detection via `is_subagent()` and `tool_name` field
- Handle `SubagentThread` content type in tool call rendering
- Add expand/collapse toggle button for subagent cards
- Style collapsed cards similar to terminal tool calls
- Support inline image rendering in content blocks

### Agent panel changes
- Add `open_external_thread_with_server()` for testing with stubbed
servers

### Test support
- Add `acp_thread/test-support` feature to agent_ui
- Add base64 dev dependency for image tests

Release Notes:

- N/A

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Co-authored-by: Amp <amp@ampcode.com>