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

cmd/simple-responder/simple-responder.go

@@ -210,6 +210,11 @@ func main() {
 		})
 	})
 
+	r.GET("/v1/audio/voices", func(c *gin.Context) {
+		model := c.Query("model")
+		c.JSON(http.StatusOK, gin.H{"voices": []string{"voice1"}, "model": model})
+	})
+
 	r.GET("/slow-respond", func(c *gin.Context) {
 		echo := c.Query("echo")
 		delay := c.Query("delay")