Improve Copilot models and dashboard toolbar

[Redth]

The Copilot model picker could remain empty or display a stale hardcoded model, while dashboard toolbar state and Release Notes presentation were inconsistent across navigation and platforms. This update makes model selection reflect the authenticated user's available models and gives the dashboard a more polished, native desktop experience.

What changed

• Load available Copilot models even when an SDK client is already connected, and display the actual selected model.
• Prefer the newest available GPT model by default, with Sol favored over Terra for equal versions and Auto as the fallback.
• Move Release Notes to a native hybrid sheet while reusing the existing Blazor release content.
• Add a labeled native macOS Release Notes toolbar control and improve development environment card borders and corner treatment.
• Reset page-scoped toolbar search, filters, text, and disabled state when navigating to a page with a new toolbar definition.
• Align the macOS GitHub Copilot SDK package version with Core.

Validation

• Added focused coverage for preferred Copilot model selection and toolbar state replacement.
• Built the macOS AppKit head and exercised the updated app locally.