defineServerAuth widens literal types despite const generic

[onmax]

Bug

defineServerAuth requires as const on string literals in additionalFields and generateId even though the generic uses const R. Without it, types like 'string' widen to string and fail to match FieldType.

Repro

// Works
export default defineServerAuth(() => ({
  advanced: { database: { generateId: 'uuid' as const } },
  user: {
    additionalFields: {
      address: { type: 'string' as const, required: false, fieldName: 'address' },
    },
  },
}))

// Fails — TS2769: Type 'string' is not assignable to type 'FieldType'
export default defineServerAuth(() => ({
  advanced: { database: { generateId: 'uuid' } },
  user: {
    additionalFields: {
      address: { type: 'string', required: false, fieldName: 'address' },
    },
  },
}))

Cause

The overload has <const R extends ServerAuthConfig> but ServerAuthConfig extends BetterAuthOptions which types deeply nested fields broadly (e.g. type: FieldType). For deeply nested properties the constraint wins over the const inference, so TypeScript widens the literal.

Suggestion

Narrow the generic to only infer the parts that matter (additionalFields, plugins, generateId) instead of constraining R against the full BetterAuthOptions. This way the const modifier can preserve literals without as const.

Version

@onmax/nuxt-better-auth v0.0.2-alpha.23