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/components/playground/ModelSelector.svelte

@@ -0,0 +1,39 @@
+<script lang="ts">
+  import { models } from "../../stores/api";
+  import { groupModels } from "../../lib/modelUtils";
+
+  interface Props {
+    value: string;
+    placeholder?: string;
+    disabled?: boolean;
+  }
+
+  let { value = $bindable(), placeholder = "Select a model...", disabled = false }: Props = $props();
+
+  let grouped = $derived(groupModels($models));
+  let hasModels = $derived(grouped.local.length > 0 || Object.keys(grouped.peersByProvider).length > 0);
+</script>
+
+{#if hasModels}
+  <select
+    class="min-w-0 flex-1 basis-48 px-3 py-2 rounded border border-gray-200 dark:border-white/10 bg-surface focus:outline-none focus:ring-2 focus:ring-primary"
+    bind:value
+    {disabled}
+  >
+    <option value="">{placeholder}</option>
+    {#if grouped.local.length > 0}
+      <optgroup label="Local">
+        {#each grouped.local as model (model.id)}
+          <option value={model.id}>{model.id}</option>
+        {/each}
+      </optgroup>
+    {/if}
+    {#each Object.entries(grouped.peersByProvider).sort(([a], [b]) => a.localeCompare(b)) as [peerId, peerModels] (peerId)}
+      <optgroup label="Peer: {peerId}">
+        {#each peerModels as model (model.id)}
+          <option value={model.id}>{model.id}</option>
+        {/each}
+      </optgroup>
+    {/each}
+  </select>
+{/if}