Fix event creation with empty user group validation and default value

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This PR fixes a critical issue where events could be created without a valid user group, resulting in inaccessible events with userGroupId=0.

Problem

The event registration form had two issues:

1. Validation bypass: Empty user group field was converted to 0 by z.coerce.number(), which passed validation but created invalid events that no one could access
2. Poor UX: No default user group was pre-selected, making it easy for users to accidentally submit with an empty field

Solution

1. Enhanced Validation

Updated the Zod schema in viewmodel.ts to ensure userGroupId is a positive integer:

userGroupId: z.coerce.number({
  required_error: 'ユーザーグループは必須です。',
}).min(1, 'ユーザーグループの指定が不正です。'),

2. Default Value Selection

Modified EventRegisterTemplate.tsx to automatically select the first available user group:

defaultValue: userGroups.length > 0 ? userGroups[0].id.toString() : undefined

Results

• Empty strings and 0 values now fail validation with proper error message: "ユーザーグループの指定が不正です。"
• First available user group is automatically pre-selected in the dropdown
• Prevents creation of inaccessible events with userGroupId=0
• Improves user experience with sensible defaults

The screenshot shows the validation error message being properly displayed when attempting to create an event with an invalid user group.

Fixes #272.

Warning

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• fonts.googleapis.com
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