- Add AgentThreadStatus (Running/Completed/Errored) and AgentThreadInfo
  to MultiWorkspace for per-workspace thread tracking
- Render thread info below each active workspace in the sidebar:
  spinning LoadCircle for running, blue check for completed, red X for errored
- Top-align folder icons in workspace entries
- Add close workspace button (X) on hover for each workspace entry
- Add remove_workspace method to MultiWorkspace
- Subscribe to AcpThread events in AgentDiff to push real-time thread
  status updates (title, stopped, error) to MultiWorkspace
- Update visual test with Completed and Running thread states

- Remove AgentThreadInfo/AgentThreadStatus types and workspace_thread_infos
  HashMap from MultiWorkspace
- Remove push_thread_info_to_multi_workspace and
  push_thread_error_to_multi_workspace from AgentDiff
- Add acp_thread and agent_ui dependencies to sidebar crate
- Sidebar now queries AgentPanel.active_agent_thread() directly to read
  thread title and status from AcpThread
- Add AgentPanelEvent::ActiveViewChanged so sidebar subscribes to targeted
  events instead of observing all panel notifications
- Sidebar subscribes to AcpThread entities (via cx.observe_in) for status
  changes and to AgentPanel events for thread switches
- Move visual test thread info setup to sidebar.set_test_thread_info()

When an agent thread is active in a workspace, its title is now persisted
to the KVP store under a single key ('sidebar-last-thread-titles') as a
JSON map keyed by sorted worktree paths. When loading recent projects in
the sidebar, thread titles are looked up from this map and displayed as
a muted subtitle beneath the project name.

- Add db and serde_json dependencies to sidebar crate
- Add sorted_paths_key() helper for canonical path-based map keys
- persist_thread_titles() writes on every queue_refresh when thread info changes
- set_recent_projects() reads the map and populates recent_project_thread_titles
- Extract render_project_row() shared by workspace and recent project entries
- Add set_test_recent_project_thread_title() for visual test injection
- Update visual test to show thread titles on two recent project entries

AgentPanel now observes the active AcpServerView entity. When the server
connection completes and the view transitions from Loading to Connected,
the AcpServerView calls cx.notify(). The AgentPanel's observation picks
this up and re-emits AgentPanelEvent::ActiveViewChanged, which the
sidebar subscribes to. This closes the timing gap where the sidebar
would refresh before the AcpThread existed and never learn about it.