fix: remove as const from defaults.ts to fix TypeScript literal type errors

The `as const` assertion caused useState to infer literal types (e.g. "0" instead of string),
preventing assignment of e.target.value (string) in onChange handlers.
Adds vitest and test coverage for defaults configuration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: T3pp31