Add /resume slash command and enhance resume picker filtering

[wangyusheng1985]

Summary

• add a /resume slash command that opens the session picker through a new AppEvent::OpenResumePicker
• enhance the picker with time-range filters, project-aware filtering via the current CWD, richer empty/
  loading states, and dynamic columns for conversation previews and relative update times
• refactor chat widget initialization so new and resumed sessions share the same setup path, and expand
  tests to cover the new resume picker behaviors

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[wangyusheng1985]

I have read the CLA Document and I hereby sign the CLA