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/lib/audioApi.ts

@@ -0,0 +1,24 @@
+import type { AudioTranscriptionResponse } from "./types";
+
+export async function transcribeAudio(
+  model: string,
+  file: File,
+  signal?: AbortSignal
+): Promise<AudioTranscriptionResponse> {
+  const formData = new FormData();
+  formData.append("file", file);
+  formData.append("model", model);
+
+  const response = await fetch("/v1/audio/transcriptions", {
+    method: "POST",
+    body: formData,
+    signal,
+  });
+
+  if (!response.ok) {
+    const errorText = await response.text();
+    throw new Error(`Audio API error: ${response.status} - ${errorText}`);
+  }
+
+  return response.json();
+}