Motivation

• Provide quick selectable presets for duplicate-finder thresholds to make tuning easier (Tight, Default, Loose).
• Detect manual edits and automatically mark the dialog as Custom so the UI reflects non-preset adjustments.
• Ensure any configuration changes are only applied to the pipeline when the user confirms by pressing OK.

Description

• Updated main_gui.py and the _open_threshold_settings dialog to add a preset_values map and import math for numeric comparison.
• Added _matches_preset and current_settings logic to detect which preset (if any) matches the loaded configuration and initialize preset_var accordingly.
• Implemented preset UI via a ttk.Combobox bound to preset_var, an _apply_preset handler that updates the dialog fields, and trace_add hooks plus _mark_custom to flip the selector to Custom when any field is edited.
• Left the existing save flow intact so the OK button (existing _save logic) remains the single point where settings are persisted and the pipeline preview/plan is cleared.

Testing

• No automated tests were run for this GUI-only change.

Codex Task