[#725] fix: implement comprehensive case-insensitive support for case import

[wayangalihpratama]

Summary

This PR implements broad case-insensitive support for the case import module and fixes a bug where adjusting segment thresholds incorrectly reset subsequent segment ranges to the global minimum.

Key Changes

• Sheet Names: Added case-insensitive sheet name resolution for "Data" and "Mapping".
• Variable Matching: Updated backend utilities (case_import_processing.py, case_import_process_confirmed_segmentation.py) and frontend components (SegmentConfigurationForm.js) to use case-insensitive matching for variable names.
• Segment Range Fix: Corrected the logic in recalculate_numerical_segments to properly identify the boundary of the previous segment regardless of variable name casing.
• Validation: Updated the validation flag lookup ("Ready for upload: YES") to be case-insensitive.

Verification

• Backend Tests: Ran tests/test_segmentation_repro.py, all passed.
• Frontend Linting: Ran yarn lint, all passed.
• Manual Verification: Try to create segment using data upload feature, then manually adjust the segment. The segment should provide correct segment range.

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Close #725

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