feat: add config.nav_links for custom header navigation links

[eclectic-coding]

Summary

• Add config.nav_links — accepts an array of { label:, url: } hashes; each entry is appended to the main navigation bar after the built-in Queue / Cache / Cable links
• Defaults to [] (no change in appearance when unconfigured)
• Remove Custom nav links from the ROADMAP v2.0 milestone

Usage

SolidStackWeb.configure do |config|
  config.nav_links = [
    { label: "Back to App", url: "/" },
    { label: "Admin",       url: "/admin" }
  ]
end

Test plan

• 410 examples, 0 failures, 100% line coverage (bundle exec rake)
• Start dummy app and verify custom links appear in the header nav after Cable
• Verify no extra links render when config.nav_links is not set

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