fix: progress bar not showing during stitch processing

setError() in the file store unconditionally set processing to false,
even when called with null to clear a previous error. This meant
setProcessing(true) followed by setError(null) at the start of
processing would immediately kill the processing state, preventing the
progress bar from ever rendering.

Fix setError to only reset processing when there is an actual error.
This also fixes the same latent bug in 10+ other custom tool settings
components that call setProcessing before setError.

apps/web/src/components/tools/stitch-settings.tsx

@@ -28,8 +28,8 @@ export function StitchSettings() {
   const handleProcess = async () => {
     if (files.length < 2) return;
 
-    setProcessing(true);
     setError(null);
+    setProcessing(true);
     setDownloadUrl(null);
     setUploadProgress(0);
 

apps/web/src/stores/file-store.ts

@@ -269,7 +269,7 @@ export const useFileStore = create<FileState>((set, get) => ({
 
   setProcessing: (v) => set({ processing: v }),
 
-  setError: (e) => set({ error: e, processing: false }),
+  setError: (e) => set(e ? { error: e, processing: false } : { error: null }),
 
   setJobId: (_id) => {
     // no-op for backward compat