Port Atlas Cloud balance provider from v0.66.0

[Finesssee]

Summary

Adds the Atlas Cloud balance provider from upstream v0.66.0 using Win-CodexBar's native Rust/Tauri patterns. It reads account-wide available USD through ATLASCLOUD_API_KEY or the existing keyring path, validates the typed API response, and registers the provider in settings and the frontend catalog. It reports the returned balance without inventing quota, spend, or history semantics.

This draft is stacked on #620 (codex/port-0.65-account-source-pack). The Atlas patch remains a single commit on that validated provider/account stack.

Validation

• The Atlas patch's focused Rust tests passed before restacking (6 passed); the original review also passed Rust Clippy and frontend tests/build.
• After restacking: cargo fmt --all -- --check and git diff --check passed.
• CircleCI Windows pr-check passed on head f9b60dfa.
• Thermo-nuclear re-review of f9b60dfa against e0a4bdc2 found no actionable maintainability issues.

Remaining evidence

• Fresh Windows-native CUA proof for the provider UI surfaces is still pending. Frontend tests/build do not replace that proof.

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