fix: more stringent checks on valid email addresses while autocompleting recipients

[madbob]

To avoid duplications I've reused the isValidEmail() function took from AccountForm.vue and moved in the global utilities file emailAddress.js

Fixes #10432

[ChristophWurst]

Thanks for the PR

[ChristophWurst]

I tried the STR from #10432 but can not reproduce on main.

How can we test this?

[madbob]

• have a Contact with an email (e.g. foobar@example.com)
• in Mail, start a new message
• in the To field, start writing "foobar@ex". The fact to add @ and one or more chars for the domain is relevant
• select the contact from the autocomplete list: this will add "foobar@example.com" to the recipients list
• blur the To field
• at this point, to the To field is appended another recipient having as email address "foobar@ex" (the partial address you were writing)

Screencast.From.2026-07-16.14-58-20.mp4

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