Bug/510796 fix revised - Refactor Invite Members Modal to use error state

What does this MR do and why?

Refactors the Invite Members Modal to improve accessibility and user experience by keeping the "Invite" button always enabled and displaying inline error validation feedback when users attempt to submit without valid input.

Related to #510796

References

Screenshots or screen recordings

Before	After	Mobile

How to set up and validate locally

Setup

1. Check out the branch

   git fetch origin
   git checkout <branch-name>

2. Install dependencies

   bundle install
   yarn install

3. Update the database (if there are migrations)

   bundle exec rails db:migrate RAILS_ENV=development

4. Start the development server

   gdk start

   Or if not using GDK:

   bundle exec rails server

5. Navigate to Project > Members > Invite Members

MR acceptance checklist

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