refactor: Migrate system preferences to field-based endpoint and remove deprecated endpoint

[angelplusultra]

Pull Request Type

• ✨ feat
• 🐛 fix
• ♻️ refactor
• 💄 style
• 🔨 chore
• 📝 docs

Relevant Issues

resolves #4957

What is in this change?

Migrates all frontend usages of the deprecated Admin.systemPreferences() service to the new field-based Admin.systemPreferencesByFields() service, then removes the deprecated endpoint and service.

Changes:

• EmbeddingTextSplitterPreference - Now requests only text_splitter_chunk_size, text_splitter_chunk_overlap, max_embed_chunk_size
• CustomSiteSettings - Now requests only meta_page_title, meta_page_favicon
• FooterCustomization - Now requests only footer_data
• ExperimentalFeatures - Now requests only feature_flags
• WorkspaceAgentConfig - Removed unnecessary preferences fetch (wasn't being used)
• Deleted GET /admin/system-preferences endpoint from server
• Deleted Admin.systemPreferences() method from frontend

This improves efficiency by only fetching the specific settings each component needs rather than fetching all system preferences.

Additional Information

The new endpoint GET /admin/system-preferences-for?labels=field1,field2 was already in place and working. This PR completes the migration by updating all consumers and removing the deprecated code.

Developer Validations

• I ran yarn lint from the root of the repo & committed changes
• Relevant documentation has been updated
• I have tested my code functionality
• Docker build succeeds locally

[shatfield4]

Fixed minor issue by destructuring settings object in CustomSiteSettings component.

LGTM