Feature Request: Support Multiple Selling Marketplaces

[VKx64]

Summary

Update the app to support multiple selling marketplaces per item, allowing users to track quantities for each marketplace separately.

Requirements

1. Excel Import:
   1. Automatically detect selling marketplaces from columns with the prefix "Selling: "
   2. Add all detected marketplaces to the app's supported list during import.
2. Item Detail Activity:
   1. Replace the "Selling Quantity" input with individual inputs for each marketplace.
   2. Display two inputs per row with the marketplace name as the label (e.g., "Amazon").
   3. Remove the old "Selling Quantity" label.
3. Item List Page:
   1. Update item descriptions to: (Total Quantity) Marketplace1: Quantity1, Marketplace2: Quantity2

Effort Estimate

Effort Level: Medium - Hard
Time Estimate: 3-5 hours