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/imageApi.ts

@@ -0,0 +1,31 @@
+import type { ImageGenerationRequest, ImageGenerationResponse } from "./types";
+
+export async function generateImage(
+  model: string,
+  prompt: string,
+  size: string,
+  signal?: AbortSignal
+): Promise<ImageGenerationResponse> {
+  const request: ImageGenerationRequest = {
+    model,
+    prompt,
+    n: 1,
+    size,
+  };
+
+  const response = await fetch("/v1/images/generations", {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/json",
+    },
+    body: JSON.stringify(request),
+    signal,
+  });
+
+  if (!response.ok) {
+    const errorText = await response.text();
+    throw new Error(`Image API error: ${response.status} - ${errorText}`);
+  }
+
+  return response.json();
+}