Add JIT-aware trading capacity helper FEDEV-3984

[qwinndev]

Summary

• Add the public GmxApiSdk.getTradingCapacity helper and typed capacity, warning, and prepare response parsing.
• Consume freshness-aware JIT snapshots in the interface, fail closed at expiry, and revalidate aged healthy snapshots immediately.
• Companion API PR: https://github.com/gmx-io/gmx-api/pull/105
• Requires the companion gmx-synthetics-keeper JIT eligibility/snapshot change before deployment; this account cannot publish that branch because direct writes and forks are disabled.
• Linear: https://linear.app/gmx-io/issue/FEDEV-3984/apisdk-expose-jit-aware-trading-capacity-and-prepare-validation

Testing

• SDK suite: 40 files, 509 tests
• Interface suite: 104 files, 956 tests; component suite: 155 tests
• SDK and interface lint, typecheck, production builds, Prettier, and diff checks
• Two independent clean review rounds on frozen cross-repository commits

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[github-actions]

Playwright Component Test Report

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