OBLS-837 Fix discrete picking QA findings

[olewandowski1]

https://openboxes.atlassian.net/browse/OBLS-837

Summary

• Search by product code works. The code was already returned by the API but never indexed, so the placeholder was accurate for names only.
• Completing a pick returns the picker to the Discrete Picking list instead of the batch "Select Pick Type and Grouping" screen. The session now records which screen it started from.
• Short picking without a reason code resets the navigation stack instead of pushing, so the finished task is no longer reachable with the back arrow. That path was pushing a screen that is not on the discrete stack, which is why the finished task stayed underneath.
• One loading indicator per state: skeleton on first load, pull to refresh control for a pull, search bar spinner for background refreshes, and no overlap with the full screen loader when an order is tapped.
• A partially picked order reads In progress with Lines: 4 / 5 Left, using the order level progress added in the backend PR.
• "Show assigned orders" is now title cased and sits closer to the queue type chips.