LeetCode 435: Non-overlapping Intervals

## [LeetCode 435: Non-overlapping Intervals](https://leetcode.com/problems/non-overlapping-intervals/)

**Difficulty:** Medium
**Category:** Intervals

### Tasks
- [ ] Implement solution in `src/intervals/non_overlapping_intervals.py`
- [ ] Verify tests pass: `pytest tests/test_non_overlapping_intervals.py`
- [ ] Analyze time and space complexity