fix(limits): preserve multi-account usage snapshots

[cbusillo]

Summary

• propagate backend rate-limit reached classifications through protocol, core, app-server, and history snapshots
• persist typed usage-limit hints per account so multi-auth account switching only blocks the limited account
• keep cached /limits snapshots visible while refreshes are due or in flight
• regenerate app-server protocol schema fixtures for the new rate-limit reached type

Validation

• cargo test -p code-core typed_usage_limit_snapshot_blocks_only_that_account
• cargo test -p code-tui limits_overlay_uses_cached_snapshot_while_refresh_due
• ./build-fast.sh
• ./pre-release.sh
• just local-code-rebuild
• manual /limits smoke confirmed by Chris