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

proxy/proxymanager_test.go

@@ -850,6 +850,43 @@ func TestProxyManager_UseModelName(t *testing.T) {
 	})
 }
 
+func TestProxyManager_AudioVoicesGETHandler(t *testing.T) {
+	conf := config.AddDefaultGroupToConfig(config.Config{
+		HealthCheckTimeout: 15,
+		Models: map[string]config.ModelConfig{
+			"model1": getTestSimpleResponderConfig("model1"),
+		},
+		LogLevel: "error",
+	})
+
+	proxy := New(conf)
+	defer proxy.StopProcesses(StopWaitForInflightRequest)
+
+	t.Run("successful GET with model query param", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/v1/audio/voices?model=model1", nil)
+		w := CreateTestResponseRecorder()
+		proxy.ServeHTTP(w, req)
+		assert.Equal(t, http.StatusOK, w.Code)
+		assert.Contains(t, w.Body.String(), "voice1")
+	})
+
+	t.Run("missing model query param returns 400", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/v1/audio/voices", nil)
+		w := CreateTestResponseRecorder()
+		proxy.ServeHTTP(w, req)
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+		assert.Contains(t, w.Body.String(), "missing required 'model' query parameter")
+	})
+
+	t.Run("unknown model returns 400", func(t *testing.T) {
+		req := httptest.NewRequest("GET", "/v1/audio/voices?model=nonexistent", nil)
+		w := CreateTestResponseRecorder()
+		proxy.ServeHTTP(w, req)
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+		assert.Contains(t, w.Body.String(), "could not find suitable handler")
+	})
+}
+
 func TestProxyManager_CORSOptionsHandler(t *testing.T) {
 	config := config.AddDefaultGroupToConfig(config.Config{
 		HealthCheckTimeout: 15,