Add SPARQL syntax validation before pasting to YASGUI

[twhetzel]

Description:
Before pasting a generated query into YASGUI, validate the SPARQL syntax:

• Use a SPARQL parser/validator library
• Show validation errors to the user
• Allow users to proceed anyway if they choose
• Highlight syntax issues in the query display

This would help catch errors before users try to execute queries in YASGUI.

Acceptance Criteria:

• SPARQL syntax validation implemented
• Clear error messages for invalid queries
• Visual indication of syntax errors
• Option to paste anyway despite errors