Add ACP agent sessions with isolated client processes

[wgqqqqq]

Summary

• Add ACP client session support across desktop, Rust ACP client management, FlowChat state, and session persistence.
• Add the ACP Agents settings page with preset setup for OpenCode, Claude Code, and Codex, including requirement probing, download/install actions, permission mode selection, and JSON config editing.
• Integrate ACP agents into workspace session creation and FlowChat streaming, including model selection, permission/tool cards, session history metadata, and external-agent session handling.
• Isolate ACP client processes per BitFun session so multiple Codex/Claude/OpenCode sessions do not share one ACP process or leak state between sessions.
• Remove ACP client auto-start/manual start semantics; ACP processes are launched from the workspace/session creation path and stopped with session/client lifecycle cleanup.
• Add a FlowChat loading indicator while an ACP session is being created, and tighten nav/sidebar overflow handling for the expanded session/workspace UI.

Behavior Notes

• The workspace menu only offers ACP session entries for enabled and runnable ACP clients.
• ACP config is normalized under acpClients; deprecated autoStart is no longer written back.
• stop_acp_client remains available to stop all running session-scoped processes for a client.
• Session-scoped ACP connections use the BitFun session id as part of the connection key to avoid cross-session process reuse.

Verification

• cargo check -p bitfun-desktop
• pnpm run type-check:web
• pnpm run lint:web
• pnpm --dir src/web-ui run test:run
• git diff --check