Model capabilities 734 (#842)

internal/config,server: implement model capabilities

- define the capabilities of a model using a simple config block on the
model
- v1/models renders out capabilities to be compatible with openrouter,
huggingface chat, and mistral formats for broader compatibility
- add support for capabilities in UI

Fixes #734

ui-svelte/src/components/playground/AudioInterface.svelte

@@ -145,7 +145,7 @@
 <div class="flex flex-col h-full">
   <!-- Model selector -->
   <div class="shrink-0 flex flex-wrap gap-2 mb-4">
-    <ModelSelector bind:value={$selectedModelStore} placeholder="Select an audio model..." disabled={isTranscribing} />
+    <ModelSelector bind:value={$selectedModelStore} placeholder="Select an audio model..." disabled={isTranscribing} capabilities={["audio_transcriptions"]} />
   </div>
 
   <!-- Empty state for no models configured -->