Optimize Media Manager sheet update trigger

- Update mediaHandlers.ts to accept an optional forcedThumbnailUrl in updateSpreadsheetThumbnail, enabling updates without re-fetching backend state.

- Update MediaManager.html execution plan to trigger the sheet update immediately (optimistically) using the predicted first item from the plan, running in parallel with other execution phases.

- Ensure the execution flow waits for both the sheet update and other operations to complete before finishing.

src/mediaHandlers.ts

@@ -119,7 +119,7 @@ export function saveMediaChanges(sku: string, finalState: any[], jobId: string |
   return logs
 }
 
-export function updateSpreadsheetThumbnail(sku: string) {
+export function updateSpreadsheetThumbnail(sku: string, forcedThumbnailUrl: string | null = null) {
   const config = new Config()
   const driveService = new GASDriveService()
   const shop = new Shop()
@@ -128,6 +128,31 @@ export function updateSpreadsheetThumbnail(sku: string) {
   const mediaService = new MediaService(driveService, shopifyMediaService, networkService, config)
 
   const ss = new GASSpreadsheetService();
+
+  // Optimization: If forced URL provided (optimistic update), skip state calculation
+  if (forcedThumbnailUrl) {
+      try {
+          const row = ss.getRowNumberByColumnValue("product_inventory", "sku", sku);
+          if (row) {
+              const thumbUrl = forcedThumbnailUrl;
+              try {
+                  const image = SpreadsheetApp.newCellImage()
+                    .setSourceUrl(thumbUrl)
+                    .setAltTextTitle(sku)
+                    .setAltTextDescription(`Thumbnail for ${sku}`)
+                    .build();
+                  ss.setCellValueByColumnName("product_inventory", row, "thumbnail", image);
+              } catch (builderErr) {
+                  ss.setCellValueByColumnName("product_inventory", row, "thumbnail", `=IMAGE("${thumbUrl}")`);
+              }
+          }
+          return;
+      } catch (e) {
+          console.warn("Failed to update sheet thumbnail (forced)", e);
+          throw new Error("Sheet Update Failed: " + e.message);
+      }
+  }
+
   const product = new Product(sku);
 
   // Need Shopify ID for accurate state logic?