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/SpeechInterface.svelte

@@ -206,7 +206,7 @@
 <div class="flex flex-col h-full">
   <!-- Model and voice selectors -->
   <div class="shrink-0 flex gap-2 mb-4">
-    <ModelSelector bind:value={$selectedModelStore} placeholder="Select a speech model..." disabled={isGenerating} />
+    <ModelSelector bind:value={$selectedModelStore} placeholder="Select a speech model..." disabled={isGenerating} capabilities={["audio_speech"]} />
     <div class="flex gap-2">
       <select
         class="shrink-0 px-3 py-2 rounded border border-gray-200 dark:border-white/10 bg-surface focus:outline-none focus:ring-2 focus:ring-primary"