Improve tree search efficiency

[foozleface]

Fixes #7752
Contributed by @foozleface

Tree search in QueryComboBox sends a LIKE query per keystroke. On tree tables with 200K+ rows, using "contains" mode generates LIKE '%pattern%' which cannot use B-tree indexes and causes full table scans. This PR changes the default search mode from "contains" to "startsWith", enabling LIKE 'pattern%' which uses B-tree indexes. It also reduces the search result limit from 1000 to 50, since a typeahead dropdown never needs that many results.

Implementation

• Change the default value of treeSearchAlgorithm user preference from contains to startsWith in UserDefinitions.tsx
• Export QUERY_COMBO_BOX_SEARCH_LIMIT constant set to 50 (was 1000)
• Add tests verifying the default search operator and result limit

Testing instructions

• Open a form with a tree-based QueryComboBox (e.g., Taxon field on a Determination)
• Type a few characters and verify the typeahead dropdown populates quickly
• Verify that typing "rosa" matches "Rosaceae" and "Rosales" but not "Pterosaurus" (startsWith behavior)
• Check user preferences -- the tree search algorithm should now default to "starts with"
• Run the frontend tests: npx jest --testPathPattern treeSearchEfficiency