Hide parent deck counts in deck browser

[omar-massfih]

Summary

Adds a new review preference to hide due counts on parent decks in the deck browser.

This is intended for users who use parent decks for organization/navigation rather than studying, where aggregated parent counts can add noise.

When enabled, parent decks hide aggregated New, Learn, and Review counts. Subdecks and leaf decks continue to display counts normally.

Notes

This change is display-only in the deck browser:

• deck tree count aggregation is unchanged
• scheduling behavior is unchanged
• default behavior is unchanged unless the preference is enabled

Testing

Added tests for:

• preference round-trip persistence
• parent vs. leaf deck display behavior