feat(desktop): add V2 agent browser pane

[Hona]

Summary

• add the advanced native browser pane to the V2 desktop/app surface
• introduce process-global BrowserHost authority with request-local, lease-fenced V2 tools
• integrate browser tools through the V2 registry, PermissionV2, normal durable settlement, and screenshot media
• keep Electron control semantic and internal, with cancellation, timeout, document-generation, and ref fencing

Architecture

Browser host state is process-global; browser tools are materialized per provider request and capture the exact Session lease/document generation. Session prompt admission and execution orchestration are unchanged.

Scope

This draft implements the local bundled sidecar host. The transport-neutral BrowserHost seam is intended for a follow-up ticketed WebSocket adapter covering WSL and remote servers; remote support is not claimed here.

Verification

• repository-wide typecheck passed in the push hook
• desktop production build and bundled server smoke passed
• focused browser host/control/tool, layout, policy, cancellation, and input-release tests passed
• changed-file lint and git diff --check passed