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.

docs/ARCHITECTURE.md

@@ -71,7 +71,15 @@ Configuration, including API keys, is stored in a dedicated Google Sheet named "
 
 ### 4. Global Entry Points (`src/global.ts`)
 
-Since Apps Script functions must be top-level to be triggered or attached to buttons, `src/global.ts` explicitly exposes necessary functions from the modules to the global scope.
+Because Webpack bundles the code into an IIFE (Immediately Invoked Function Expression) to avoid global scope pollution, top-level functions defined in modules are **not** automatically globally accessible in the Apps Script environment.
+
+- **Requirement**: Any function that needs to be called from the frontend via `google.script.run`, triggered by a menu, or attached to a spreadsheet event must be explicitly assigned to the `global` object in `src/global.ts`.
+- **Example**:
+  ```typescript
+  import { myFunc } from "./myModule"
+  ;(global as any).myFunc = myFunc
+  ```
+- **Rationale**: This is the only way for the Google Apps Script runtime to find these functions when they are invoked via the `google.script.run` API or other entry point mechanisms.
 
 ### 5. Status Automation (`src/statusHandlers.ts`)