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.
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