ui: add Rerank tab to playground (#536)

Add a new Rerank tab to the playground that lets users test /v1/rerank
endpoints. Supports a visual table editor and a JSON editor mode that
stay in sync when toggling between them.

- add rerankApi.ts with typed wrapper for /v1/rerank
- add RerankInterface.svelte with query input, sortable document table,
color-coded scores, auto-add row, cancel/clear, and token usage
- add rerankLoading store to playgroundActivity derived store
- register Rerank tab in Playground.svelte

Updates #481

ui-svelte/src/lib/rerankApi.ts

@@ -0,0 +1,27 @@
+export interface RerankResult {
+  index: number;
+  relevance_score: number;
+}
+
+export interface RerankResponse {
+  model: string;
+  object: string;
+  usage: { prompt_tokens: number; total_tokens: number };
+  results: RerankResult[];
+}
+
+export async function rerank(
+  model: string,
+  query: string,
+  documents: string[],
+  signal: AbortSignal
+): Promise<RerankResponse> {
+  const response = await fetch("/v1/rerank", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({ model, query, documents }),
+    signal,
+  });
+  if (!response.ok) throw new Error(`${response.status} ${response.statusText}`);
+  return response.json();
+}