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/mediaManager.integration.test.ts

@@ -12,7 +12,8 @@ const mockDrive = {
     trashFile: jest.fn(),
     updateFileProperties: jest.fn(),
     getFileProperties: jest.fn(),
-    getFileById: jest.fn()
+    getFileById: jest.fn(),
+    getFilesWithProperties: jest.fn()
 }
 const mockShopify = {
     getProductMedia: jest.fn(),
@@ -80,6 +81,13 @@ describe("MediaService V2 Integration Logic", () => {
             getId: () => "new_created_file_id"
         })
         mockDrive.getFileProperties.mockReturnValue({})
+        mockDrive.getFilesWithProperties.mockImplementation((folderId: string) => {
+            const files = mockDrive.getFiles(folderId) || []
+            return files.map(f => ({
+                file: f,
+                properties: mockDrive.getFileProperties(f.getId())
+            }))
+        })
     })
 
     describe("getUnifiedMediaState (Phase A)", () => {