fix(dashboard): preserve large manifest quotas - #6802

Summary

• parse TOML integers as BigInt only when JavaScript Number cannot represent them safely
• preserve large resource quota values as validated decimal strings during form serialization
• continue rejecting negatives, non-integers, and values beyond TOML's signed 64-bit integer range

Verification

• pnpm exec vitest run src/lib/agentManifest.test.ts (25 tests)
• pnpm test (91 files, 952 tests)
• pnpm typecheck
• pnpm lint
• pnpm build