Restore email scoped label styles

What does this MR do and why?

Closes Restore email scoped label styles (#627685 - closed); the css_parser upgrade (!250166 (merged)) fixed a parsing bug.. which meant our scoped labels started to look weird.

Fix them:

• Use a CSS border to draw their borders; Gmail drops box-shadows entirely, so using a plain border is reliable.
• But then they look too bloated, since it's 2px extra on all sides. Drop that same amount from the padding, but only in emails.

Screenshots or screen recordings

Comment in question in the web:

Before	border only	After (border with padding compensation)

How to set up and validate locally

1. To see this in Gmail, you need to drop something like this in your config/initializers/smtp_settings.rb:

   if Rails.env.development?
     ActionMailer::Base.delivery_method = :smtp
     ActionMailer::Base.smtp_settings = {
       address: 'YOUR_SMTP_SERVER',
       port: 587,
       user_name: 'YOUR_USERNAME',
       password: 'YOUR_PASSWORD',
       authentication: :plain,
       enable_starttls_auto: true
     }
   
     redirect_all_mail = Class.new do
       def self.delivering_email(message)
         message.to = ['YOU+gdk-dest@gitlab.com']
         message.from = ['YOUR_EMAIL_ADDRESS']
         message.cc = nil
         message.bcc = nil
       end
     end
     ActionMailer::Base.register_interceptor(redirect_all_mail)
   end

   Restart GDK after dropping that in.

   If not, you can test against LetterOpener (/rails/letter_opener), but I've been using Gmail for highest-fidelity, since our reports have been coming internally.

2. With master checked out, write a note that includes a few scoped labels (like mine). They're an EE-only feature.

3. Drop this into a file called scripts/frontend/_tmp.mjs:

   import { compileAllStyles } from './lib/compile_css.mjs';
   await compileAllStyles({ filter: (p) => p.includes('/mailers/'), style: 'compressed' });

   Then, run node scripts/frontend/_tmp.mjs. This precompiles your styles, ready for Premailer, the same way it's done for prod.

4. Then, open a Rails console, and issue:

   ViteRuby.env['VITE_ENABLED'] = 'false'
   Notify.note_issue_email(User.find_by_username('root').id, Note.last.id).deliver_now

   The first line is to disable Vite in your console session, in case it's on. Premailer doesn't jive with Vite and you won't get any working stylesheet styles in your emails at all like that, only inline ones.

   The second line triggers the email for the last-created note.

5. Wait for your email or refresh LetterOpener. It should look like "Before".

6. Check out this branch. Re-do steps 3 through 5.

7. It should look like "After".

8. Tell me your favourite Panthera. The reproduction steps aren't complete until you've done this.

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