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/stores/playgroundActivity.ts

@@ -4,10 +4,11 @@ const chatStreaming = writable(false);
 const imageGenerating = writable(false);
 const speechGenerating = writable(false);
 const audioTranscribing = writable(false);
+const rerankLoading = writable(false);
 
 export const playgroundActivity = derived(
-  [chatStreaming, imageGenerating, speechGenerating, audioTranscribing],
-  ([$chat, $image, $speech, $audio]) => $chat || $image || $speech || $audio
+  [chatStreaming, imageGenerating, speechGenerating, audioTranscribing, rerankLoading],
+  ([$chat, $image, $speech, $audio, $rerank]) => $chat || $image || $speech || $audio || $rerank
 );
 
 export const playgroundStores = {
@@ -15,4 +16,5 @@ export const playgroundStores = {
   imageGenerating,
   speechGenerating,
   audioTranscribing,
+  rerankLoading,
 };