Add Claude Fable 5 model option

Adds the new claude-fable-5 model (released 2026-06-09) as a selectable
model alongside the existing Claude models:

- ChatService.ResolveModelId: map "fable" and "fable-5" to claude-fable-5
- index.html: fable-5 option in the model dropdown (after opus-4-8)
- ARCHITECTURE.md: model list + resolution table updated

Default stays opus-4-8. The invite-only sibling Mythos 5 is intentionally
not included.

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

Services/ChatService.cs

@@ -49,6 +49,8 @@ public class ChatService
             "opus-4-6" => "claude-opus-4-6",
             "opus-4-7" => "claude-opus-4-7",
             "opus-4-8" => "claude-opus-4-8",
+            "fable" => "claude-fable-5",
+            "fable-5" => "claude-fable-5",
             "haiku" => "claude-haiku-4-5",
             "haiku-4-5" => "claude-haiku-4-5",
             _ => "claude-sonnet-4-6"