Add read-only SnapTrade broker pack

[MasashiToda1]

Summary

• Add a signed SnapTrade Personal API read client and optional UTA broker pack.
• Surface a read-only SnapTrade preset for securities monitoring.
• Re-check connection status and require realtime/read/not-disabled before every account read.
• Permanently reject every order mutation before network I/O.

Scope

Initial position mapping is stock-like securities only: stocks, ETFs, ADRs, CEFs, and mutual funds. Unsupported derivatives, crypto, and cash-equivalents loud-refuse rather than being misclassified.

Validation

• SnapTrade broker test suite passed: 9 tests.
• TypeScript check passed.
• SnapTrade broker-pack build passed.

The repository-wide test command also ran. Existing environment-sensitive failures remain: local shellPath expectation, app-resource pack path, and UI localstorage/WebGL tests.

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