feat(media): Optimize Media Manager loading performance

Significant performance improvements to the 'Loading media...' phase:
- Reduced client-server round trips by consolidating the initial handshake (diagnostics + media fetch) into a single backend call: getMediaManagerInitialState.
- Implemented batched Google Drive metadata retrieval in GASDriveService using the Advanced Drive API, eliminating per-file property fetching calls.
- Switched to HtmlService templates in showMediaManager to pass initial SKU/Title data directly, enabling the UI shell to appear instantly upon opening.
- Updated documentation (ARCHITECTURE.md, MEMORY.md) to clarify Webpack global assignment requirements for GAS functions.
- Verified with comprehensive updates to unit and integration tests.

src/services/MockDriveService.ts

@@ -127,4 +127,12 @@ export class MockDriveService implements IDriveService {
           return {}
       }
   }
+
+  getFilesWithProperties(folderId: string): { file: GoogleAppsScript.Drive.File, properties: { [key: string]: string } }[] {
+    const files = this.getFiles(folderId)
+    return files.map(f => ({
+      file: f,
+      properties: (f as any)._properties || {}
+    }))
+  }
 }