Desktop: Plan feature not tested; plan output lacks special renderer

[wangtsiao]

Task

The Plan feature has not been tested on the Desktop app.

Issues

1. No Desktop testing — The /plan or plan feature hasn't been tested in the Desktop app
2. No special renderer — Plan output is rendered as plain text rather than with a dedicated UI that shows steps, architecture, and dependencies in a structured way

Expected behavior

• Plan output should have a dedicated renderer or at minimum be clearly readable on Desktop
• End-to-end testing of plan creation, display, and execution flow on Desktop

Related components

• crates/protocol/src/acp_session_mode.rs — session mode for planning
• Desktop app chat view components