feat(icon): make the chrome icon library configurable via DocsKit.configuration.icon_library

[mhenrixon]

Problem

DocsUI::Icon renders the docs chrome's lucide icon names (menu, search, file-code, list, info, palette, lightbulb, triangle-alert, file-text, ...) but resolved them against RailsIcons.configuration.default_library.

On a host app whose default_library is not lucide (e.g. phosphor or heroicons), those lucide names don't exist in the configured library, so every chrome icon raises Icons::IconNotFound in development. The host app couldn't fix this without flipping its global RailsIcons default — which it shouldn't have to do just to render docs-kit's chrome.

Fix

Add a DocsKit.configuration.icon_library config option (default "lucide", matching the lucide icon names docs-kit ships). DocsUI::Icon#rails_icons_library now reads it, falling back to the host app's RailsIcons.configuration.default_library:

def rails_icons_library
  DocsKit.configuration.icon_library || ::RailsIcons.configuration.default_library
rescue StandardError
  nil
end

• Zero-config, backwards compatible — the default is "lucide", and the icon names docs-kit ships are lucide names, so existing lucide-default apps are unaffected.
• Independent of the host's global default — a phosphor/heroicons app leaves icon_library at "lucide" and the chrome keeps rendering, without changing its own default_library.
• Escape hatch — set icon_library to nil to restore the old defer-to-host behavior.

This follows the project invariant that site-specific values live on DocsKit.configuration with a sensible default.

Test plan

New specs (written first, RED → GREEN):

• spec/docs_kit/configuration_spec.rb — #icon_library defaults to "lucide"; overridable via DocsKit.configure { |c| c.icon_library = "phosphor" }.
• spec/docs_ui/icon_spec.rb — #rails_icons_library:
  • returns the configured icon_library when set (the || short-circuits, so it never touches RailsIcons — the representative path for a non-lucide host);
  • falls back to RailsIcons.configuration.default_library when icon_library is nil (RailsIcons stubbed in — it's a Railtie gem, absent from the isolated suite);
  • degrades to nil when RailsIcons isn't loaded.

Verification (local — this repo has no PR CI, only a release-triggered deploy workflow):

• bundle exec rspec → 55 examples, 0 failures
• bundle exec rubocop lib app spec → 32 files, no offenses

Closes #3