Mark Apple Intelligence provider as Beta throughout the UI

Apple's Foundation Models framework is still under active development
(built against macOS 27 beta) and likely to stay rough for a while —
small 4K context window, occasional generation errors, no tool
support yet. Surface that clearly wherever it appears:
- Model name: "Apple On-Device (Beta)" (shows in header/model picker)
- Model description: notes the beta status and what to expect
- Settings -> General: "⚠️ Beta — ..." disclaimer under the Apple
  Intelligence section, same style as the existing Paperless-NGX beta
  note
- Credits panel: same disclaimer for the Apple Intelligence entry

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

oAI/Providers/AppleFoundationProvider.swift

@@ -42,8 +42,8 @@ final class AppleFoundationProvider: AIProvider {
         [
             ModelInfo(
                 id: "apple-on-device",
-                name: "Apple On-Device",
-                description: "On-device Apple Intelligence model. Private, free, and works offline. 4K context window.",
+                name: "Apple On-Device (Beta)",
+                description: "On-device Apple Intelligence model. Private, free, and works offline. 4K context window. Apple's Foundation Models framework is still in active beta (currently macOS 27 beta) — expect occasional generation errors and rough edges.",
                 contextLength: 4096,
                 pricing: ModelInfo.Pricing(prompt: 0, completion: 0),
                 capabilities: ModelInfo.ModelCapabilities(