feat: add sub2api provider (upstream 0.43.0)

[Finesssee]

Summary

Port sub2api from upstream CodexBar v0.43.0 into Win-CodexBar (PR A of the 0.43.0 port stack).

• New provider: GET {base}/v1/usage with Bearer group/fallback API key
• Env: SUB2API_API_KEY, SUB2API_BASE_URL (HTTPS or loopback HTTP)
• Maps keyQuota / subscription (daily/weekly/monthly) / wallet + rate_limits
• Token accounts for group API keys; settings workspace base URL
• Icons + frontend catalog wiring
• Unit tests (10) for parse, URL join, base URL validation

Version

Still 0.42.0 marketing version (packaging PR F comes last).

Validation

• cargo test --manifest-path rust/Cargo.toml sub2api — 10 passed
• test_provider_id_all / every_provider_id_is_instantiable — ok (per implementer)

Plan

Part of Windows 0.43.0 port. Next: account-scoped notifications, Factory API-key Auto, cost/stale refresh, P1 residuals, then version bump.