Add support for scoped subitem search using > prefix

[mmheyer]

Implements feature request #1885 by enabling users to scope searches to a specific item and its subitems using a > prefix (e.g., >Lectures design patterns). Includes:

• Query parsing and scoping logic
• UI message update for scoped searches
• Unit tests for query parsing

RESOLVES #1885

[RogerHaase]

@mmheyer did you notice you have a failing test?

Found another search bug #1907. OK to fix this PR, work on 1907 later or leave for someone else.