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/playground/PlaceholderTab.svelte

@@ -0,0 +1,14 @@
+<script lang="ts">
+  interface Props {
+    featureName: string;
+  }
+
+  let { featureName }: Props = $props();
+</script>
+
+<div class="flex items-center justify-center h-full">
+  <div class="text-center text-txtsecondary">
+    <p class="text-lg">{featureName}</p>
+    <p class="text-sm mt-2">To be implemented</p>
+  </div>
+</div>