Fixes launch freeze by running TripPlanningEngine in background task.
UI now responsive immediately while trips generate.
- Move heavy computation to Task.detached with .userInitiated priority
- Change all helper methods to nonisolated static functions
- Create local TripPlanningEngine in background task instead of instance property
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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