feat: add config.dashboard_cards for custom overview dashboard cards

[eclectic-coding]

Summary

• Add config.dashboard_cards — accepts an array of { title:, link:, stats: } hashes; each entry renders a custom card on the overview dashboard after the built-in Queue, Cache, and Cable cards
• stats is an optional lambda returning a { label => value } hash, evaluated at render time — host apps can call their own models here
• link is an optional { label:, url: } hash rendered as the card header link
• Custom cards get a neutral grey top-border (sqw-gem-card--custom) so they're visually distinct from the built-in coloured cards
• Defaults to [] — no visible change when unconfigured
• Removes the completed v2.0 milestone section from ROADMAP (both Custom nav links and Custom dashboard cards are now shipped)

Usage

SolidStackWeb.configure do |config|
  config.dashboard_cards = [
    {
      title: "My App",
      link:  { label: "View Admin", url: "/admin" },
      stats: -> { { "Users" => User.count, "Premium" => User.premium.count } }
    }
  ]
end

Test plan

• 415 examples, 0 failures, 100% line coverage (bundle exec rake)
• Start dummy app and verify no extra cards with default config
• Configure a card with stats lambda and verify it renders on the dashboard

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