feat: add additionalDirectories support in project settings

[Frenvius]

Summary

• Add UI in Project Settings to manage external directories that Claude can access outside the worktree, stored in .claude/settings.local.json under permissions.additionalDirectories
• Add Tauri commands (get_project_additional_dirs, set_project_additional_dirs) to read/write the setting with validation that paths exist and are directories
• Auto-sync additional directories from the project's settings to each worktree's .claude/settings.local.json when launching Claude CLI