Add Codex session manager for cross-instance sync and trash

[GiZGY]

Summary

This PR adds a dedicated Codex session management workspace for multi-instance workflows.

Users can now:

• browse Codex sessions across all managed instances from a standalone Sessions tab
• sync missing sessions and threads across instances with one click
• move selected sessions to Trash for safer cleanup
• manage sessions in a grouped, collapsible layout closer to Codex's native thread list

What changed

• added a standalone Sessions tab under Codex account management
• added backend commands to:
  • list sessions across Codex instances
  • sync missing threads and sessions across instances
  • move selected sessions to Trash across instances
• added frontend store, service, and types for Codex session management
• added grouped session UI by working directory
• kept groups collapsed by default so users expand manually
• fixed session-page refresh overlap and expansion-state reset issues
• added localized UI copy for the new session management flow

Scope

This PR is intentionally scoped to Codex multi-instance and session management only.
No non-Codex platform business logic was changed.

Validation

Ran locally:

• npm install
• npm run typecheck
• node scripts/check_locales.cjs
• cargo check

Manual checks on macOS:

• Sessions tab renders correctly
• session groups stay collapsed by default
• expanding or collapsing one group no longer re-expands others unexpectedly
• sync action writes missing sessions into other instances
• move-to-trash action removes selected sessions from the manager list

Notes

• Running target instances may still require restart before Codex itself reflects updated session or thread data.
• Screenshot for this PR should use the masked session-manager view from local validation.

[GiZGY]