feat: add automatic archiving for inactive unpinned chats

[NickTitle]

Summary

• add an automatic archive duration picker under Settings → Archive, defaulting to Never
• require explicit confirmation before enabling automatic archiving
• sweep inactive chats on startup, hourly, when the app returns to the foreground, and when the setting changes
• protect pinned, active, running, windowed, recently active, or draft-bearing chats
• wait for persisted queues and window snapshots to hydrate before archiving
• preserve worktrees and branches during automatic archival
• isolate per-session failures and safely handle paged-out sessions
• add English and Spanish copy plus preference, safety, selection, and UI coverage

Testing

• pnpm check
• pnpm typecheck
• 233 focused Vitest tests
• pre-push hooks: tauri-check, clippy, fmt-check, check

Prior review

Port of squareup/berd#1137, including all human, Builderbot, and adversarial-review follow-up fixes.