fix(vibenet): preserve sign when formatting token amounts

[burak33bb]

Summary

• Fix negative fractional output in app/vibenet/library/format.ts's string-based formatAmount helper.
• Add focused regression tests for positive, negative, and invalid amount inputs.

Why

This is separate from #68, which updates fmtTokenAmount in app/vibenet/library/explorer.ts and formatBigInt in app/tips/library/format.ts; this PR fixes the Vibenet faucet/status formatter used by faucetTokens.ts.

Testing

• npx vitest run app/vibenet/library/format.test.ts
• npm run typecheck
• npm run lint
• git diff --check

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[cb-heimdall]

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