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.

MEMORY.md

@@ -18,7 +18,7 @@ This project (`product_inventory`) integrates Google Sheets with Shopify. It ser
 ## Key Technical Decisions
 - **Queue System**: We implemented `onEditQueue.ts` to batch edits. This prevents hitting Shopify API rate limits and Google Apps Script execution limits during rapid manual edits.
 - **Hybrid API**: We use REST for retrieving Orders (legacy/easier for flat data) and GraphQL for Products (more efficient/flexible).
-- **Global Exports**: Functions in `src/global.ts` are explicitly exposed to be callable by Apps Script triggers.
+- **Global Exports**: Functions in `src/global.ts` must be explicitly assigned to the `global` object (e.g., `(global as any).func = func`). This is required because Webpack bundles code into an IIFE, making top-level module functions unreachable from the frontend `google.script.run` or Apps Script triggers unless exposed this way.
 
 ## User Preferences
 - **OS**: Windows.