fix: Harmonize SSO Buttons

[AyakorK]

🎩 Description

Unifies the SSO login button override mechanism across all omniauth providers. Previously each provider had its own strategy: FranceConnect hardcoded an image-only partial, Publik hacked the shared log_in_with_provider Term Customizer key, others only checked display_name, and the default fallback truncated the provider name ("France" instead of "France Connect").

Now every provider goes through one cascade in omniauth_provider_label: a per-provider Term Customizer translation → display_name -> a proper humanized name. Icon handling is unified too: any provider with icon_path + icon_hover_path configured gets the full-image button treatment, and a safe generic icon fallback prevents crashes when nothing is configured, including a real NoMethodError crash in decidim-core's oauth_icon for any provider not registered in Decidim.omniauth_providers.

Testing

• Activate the SSOs you want to test in the system view
• Visit the login/registration page and confirm every button renders without error (icon or fallback icon)
• Confirm FranceConnect still renders as a full image button, and other providers show a proper full name ("Cultuur Connect", not "Cultuur")
• In Term Customizer, override decidim.devise.shared.links.provider_display_name_<provider> for a provider, run the GLOBAL cache cleaner (the term customizer "cache cleaner" button only clears the participatory processes related so it might not work), and confirm the button text updates

📌 Related Issues

• Fixes https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?filterQuery=assignee%3AAyakorK&pane=issue&itemId=210192975&issue=OpenSourcePolitics%7Cintern-tasks%7C462

Tasks

• Added specs for the provider label cascade and icon fallback logic (omniauth_helper_spec.rb)
• Added a system spec confirming every configured omniauth button renders without a server error
• Registered log_in_with_provider and provider_display_name in config/i18n-tasks.yml so they aren't flagged as unused, since they're referenced dynamically with an interpolated provider name rather than called literally in the code