webui: fix UI freeze at high token rates with RAF yield

The markdown coalescing loop was processing chunks back-to-back without
yielding to the browser's paint cycle. At high token rates (250+ tok/s),
this caused complete UI freeze as the main thread was perpetually busy.

Add a requestAnimationFrame yield between processing batches. This allows
the browser to paint at screen FPS regardless of token throughput. Chunks
arriving during the yield are coalesced and processed together, so we
skip intermediate states and jump straight to the latest content.

Before: Chunk->process->Chunk->process->... (browser never paints = freeze)
After:  Chunk->process->[RAF]->coalesced chunks->process->[RAF]->... (screen FPS)

Tested with 250 tok/s streams on 50K+ token contexts: smooth scrolling
and responsive UI throughout.

tools/server/webui/src/lib/components/app/content/MarkdownContent.svelte

@@ -499,6 +499,7 @@
 	/**
 	 * Queues markdown for processing with coalescing support.
 	 * Only processes the latest markdown when multiple updates arrive quickly.
+	 * Uses requestAnimationFrame to yield to browser paint between batches.
 	 * @param markdown - The markdown content to render
 	 */
 	async function updateRenderedBlocks(markdown: string) {
@@ -516,6 +517,12 @@
 				pendingMarkdown = null;
 
 				await processMarkdown(nextMarkdown);
+
+				// Yield to browser for paint before processing next batch
+				// This prevents UI freeze at high token rates (250+ tok/s)
+				if (pendingMarkdown !== null) {
+					await new Promise((resolve) => requestAnimationFrame(resolve));
+				}
 			}
 		} catch (error) {
 			console.error('Failed to process markdown:', error);