feat(ui): raise Settings window and filter provider order to configured

[victorhqc]

Summary

Fixes two Settings-window papercuts: re-clicking "Settings…" now raises the already-open window instead of leaving it behind the frontmost app, and the Appearance tab's "Provider order" list shows only providers that are actually configured.

Why

Two user-facing bugs (ui 16). The Settings window could get lost behind other apps when re-opened, and the provider order list included every registered provider regardless of whether the user had set it up.

What changed

• Settings window activation. Replaced SettingsLink with a custom button modifier that calls openSettings(), activates Filbert, and explicitly raises the Settings NSWindow. The window lookup uses structural traits (styleMask.contains(.titled) and not NSPanel) instead of localized title text, so it works across locales.
• Configured-only provider order. Added configuredProvidersOrdered to QuotaViewModel, filtered by the same state predicate used in refreshDerived(). The Appearance tab now renders this filtered list and shows an empty-state hint when no providers are configured.
• Drag-drop index mapping. The move helper resolves visible indices against the full ordered list, so saved positions of unconfigured providers are preserved when they re-appear later.
• Tests. Unit tests for configuredProvidersOrdered across mixed configured/unconfigured registries.
• Spec. New spec (ui 16) covering both issues.
• Writing-code skill docs. Minor clarification around AC marker usage.

Notes for review

• The window-raise behavior is not unit-testable without a host app — covered by manual verification per the spec ACs.
• Drag-drop on a filtered list moves by provider ID, not visible index, so hidden providers stay put. Worth a quick manual check after review.