Add /resume (session resume) to desktop app

Summary

The CLI supports resuming prior sessions via claude --resume / claude --continue / /resume, but the desktop app has no equivalent. There's no /resume slash command and no session picker UI.

Request

Bring session resume to the desktop app at parity with the CLI:

• /resume slash command, or
• A session history picker (list prior sessions, select to resume with full context)
• Support for --continue equivalent (resume most recent session in the current working directory)

Why

• CLI and desktop currently diverge on a core workflow feature.
• Users who switch between CLI and desktop lose the ability to pick up prior work when in the desktop app.
• Session history already appears to be saved — exposing it in the UI should be mostly a surfacing problem.

Environment

• Claude Code desktop app (latest)