fix: newsletter-banner-color

[BarbaraOliveira13]

🎩 Description

Backport of the newsletter banner color fix from #954 (branch alt/cd44) to rc-4.0.0.

Instead of hardcoding the white color, this PR introduces an environment variable DECIDIM_CUSTOM_NEWSLETTER_BAR_COLOR to configure the banner color per client. If the variable is not set, the default Decidim behavior is preserved — no other clients are impacted.

The variable must be set to #FFFFFF for Loire Atlantique/cd44 after merging.

Also, fix Accessibilty test spec/system/proposals_index_spec.rb[1:1:2:2] by override .rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/bundler/gems/decidim-a664bb881843/decidim-core/app/cells/decidim/data_consent/category.erb app/cells/decidim/data_consent/category.erb and changing h3 -> span Line 16

Testing

• Set DECIDIM_CUSTOM_NEWSLETTER_BAR_COLOR="#FFFFFF" in your .env or when you're run 'DECIDIM_CUSTOM_NEWSLETTER_BAR_COLOR=#FFFFFF bundle exec rails s'
• Send a test newsletter from the admin panel
• Check that the banner background color is white
• Remove the variable and verify the banner returns to its default color

📌 Related Issues

• Fixes #409

Tasks

• Add specs
• In case of new dependencies or version bump, update related documentation

🛠️ If your PR introduces or updates an Override

• Add the label "override:pending" in your PR
• Add the corresponding label based on the review deadline category (when this override must be checked):
  • WHEN BACKPORT 0.29 ·
  • BEFORE MIGRATION 0.31 ·
  • WAITING FOR PR UPSTREAM ·
  • OTHER
• Add the link to the related Decidim issue or PR (if exist).

📷 Screenshots

Please add screenshots of the changes you're proposing if related to the UI

Extra information

[Stef-Rousset]

LGTM !