perf(ui): update the download progress once per visible change

fetchWithProgress reports every network chunk; only re-render when the
"x MB / y MB" label actually changes (#1).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/App.tsx

@@ -152,10 +152,14 @@ export function App() {
         let name: string
         if (source.kind === "url") {
           name = source.name ?? nameFromUrl(source.url)
+          let lastLabel = ""
           buf = await fetchWithProgress(source.url, (loaded, total) => {
             if (stale()) return
             const size = total ? `${formatBytes(loaded)} / ${formatBytes(total)}` : formatBytes(loaded)
-            setLoadState({ label: `도면 내려받는 중 (${size})`, value: total ? Math.round(Math.min(1, loaded / total) * 50) : null, busy: true })
+            const label = `도면 내려받는 중 (${size})`
+            if (label === lastLabel) return // one update per visible change, not per network chunk
+            lastLabel = label
+            setLoadState({ label, value: total ? Math.round(Math.min(1, loaded / total) * 50) : null, busy: true })
           })
         } else {
           name = source.file.name