proxy,ui-svelte: replace old UI with svelte+playground

Replace the legacy React UI with the new Svelte-based one. Introduce a Playground in the UI to quickly test out text, image, text to speech and speech to text models behind llama-swap. 

Key Changes

New Svelte UI (ui-svelte/)

  - Multi-tab Playground with Chat, Image Generation, Audio Transcription, and Speech interfaces
  - Chat: message editing/regeneration, markdown rendering with LaTeX math support, image attachments, code syntax highlighting
  - Image: size selector, download/fullscreen viewing
  - Audio: transcription with peer support
  - Speech: voice caching with manual refresh, download button
  - Responsive mobile layout with collapsible navigation
  - XSS fixes and accessibility improvements

Proxy Improvements

  - Add gzip/brotli compression for UI static assets (proxy/ui_compress.go)
  - Add GET /v1/audio/voices?model={model} endpoint for voice listing
  - Add peer support for /v1/audio/transcriptions

ui-svelte/src/components/Header.svelte

@@ -43,19 +43,27 @@
     </h1>
   {/if}
 
-  <menu class="flex items-center gap-4">
+  <menu class="flex items-center gap-4 overflow-x-auto">
     <a
       href="/"
       use:link
-      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1"
+      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1 whitespace-nowrap"
       class:font-semibold={isActive("/", $location)}
+    >
+      Playground
+    </a>
+    <a
+      href="/models"
+      use:link
+      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1 whitespace-nowrap"
+      class:font-semibold={isActive("/models", $location)}
     >
       Models
     </a>
     <a
       href="/activity"
       use:link
-      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1"
+      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1 whitespace-nowrap"
       class:font-semibold={isActive("/activity", $location)}
     >
       Activity
@@ -63,7 +71,7 @@
     <a
       href="/logs"
       use:link
-      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1"
+      class="text-gray-600 hover:text-black dark:text-gray-300 dark:hover:text-gray-100 p-1 whitespace-nowrap"
       class:font-semibold={isActive("/logs", $location)}
     >
       Logs