Phase 4 COMPLETE: Modern GUI, File Picker, Charts

- FULL GUI Redesign complete.
- Implemented SORTABLE tables for Level Counts and Top Errors.
- Added CUSTOM FILE PICKER using std::filesystem (zero external deps).
- Added LEVEL DISTRIBUTION CHART using custom ImDrawList rendering.
- Modern Dark Theme with Coral accents fully applied.
- Threading and Cancel support verified.

Technical details:
- GuiController.cpp: Added sort specs handling for ImGui tables.
- GuiController.cpp: Implemented directory walking and modal popup for file picker.
- GuiController.cpp: Custom geometry drawing for bar chart.

Ready for Phase 5 (CI/CD).

Author: parvenuprompting