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>

wwwroot/index.html

@@ -205,6 +205,7 @@
           <option value="opus-4-6">opus-4-6</option>
           <option value="opus-4-7">opus-4-7</option>
           <option value="opus-4-8" selected>opus-4-8</option>
+          <option value="fable-5">fable-5</option>
           <option value="haiku-4-5">haiku-4-5</option>
           <optgroup label="OpenAI" id="openaiOptgroup">
             <option value="" disabled>Wird geladen...</option>