fix: render fresh upload progress as empty

A newly selected file has no server upload record yet, so the UI calls the
progress renderer with zero completed chunks and zero total chunks. Treating
that zero-total state as complete made the progress bar jump to 100% before
any upload had started.

Render zero-total progress as empty instead. Existing resumable uploads still
show their server-authoritative completed chunk percentage, and completed
non-empty uploads still render as full because their completed count equals a
non-zero total.

Test Plan:
- just static-check
- just test
- git diff --check

static/app.js

@@ -198,7 +198,7 @@ function renderButtons() {
 }
 
 function updateProgress(completedCount, totalChunks) {
-  const percentage = totalChunks === 0 ? 100 : (completedCount / totalChunks) * 100;
+  const percentage = totalChunks === 0 ? 0 : (completedCount / totalChunks) * 100;
   progressBar.style.width = `${percentage}%`;
   progressMeta.textContent = `${completedCount} of ${totalChunks} chunks`;
 }