fix(electron): resolve packaged app errors and XML schema validation

- Remove preload script configuration causing DOM access errors
- Wrap DOM access in DOMContentLoaded event handler
- Fix off-by-one loop error in pushLearners (i <= length → i < length)
- Restructure learner object to match exact ILR schema element order
- Split buildLearnerDetails into personal and contact details
- Split buildHealthDetails to position LLDDHealthProb before NINumber
- Add error handling for CSV parsing and file selection
- Remove example CSV files from docs/inputs

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Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: JasonWarrenUK