feat: manage broker accounts per session

[atyrode]

Summary

• replace profile-based vault switching with immutable per-launch OAuth account allowlists
• add account manager presets, enable/disable controls, usage-aware account display, aligned usage geometry, reset-credit details, and stale-cache retention
• pin the bundled OMP to the fork release that enforces allowlists in credential resolution, refresh, usage, and broker snapshots

Verification

• nix shell nixpkgs#go nixpkgs#gcc -c go test ./...
• nix build .#with-omp
• ./result/bin/omp --version
• ./result/bin/omp --smoke-test
• interactive TUI previews at 120x52, including compact, account-manager, loading, stale, aggregate, and side-by-side layouts