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/App.svelte

@@ -5,14 +5,16 @@
   import LogViewer from "./routes/LogViewer.svelte";
   import Models from "./routes/Models.svelte";
   import Activity from "./routes/Activity.svelte";
+  import Playground from "./routes/Playground.svelte";
   import { enableAPIEvents } from "./stores/api";
   import { initScreenWidth, isDarkMode, appTitle, connectionState } from "./stores/theme";
 
   const routes = {
-    "/": Models,
+    "/": Playground,
+    "/models": Models,
     "/logs": LogViewer,
     "/activity": Activity,
-    "*": Models,
+    "*": Playground,
   };
 
   // Sync theme to document attribute