Fix ANR, false tack detections, and empty-state flash on track load

ANR: getSavedTracks() had no IO dispatcher — TackDetector ran on main
thread at O(n²) cost (~77M ops for a 2.5h track). Added withContext(IO).

TackDetector: replaced O(n) list.filter with O(log n) binary-search
subList for before/after windows; O(n²) → O(n log n). Raised MIN_DELTA
60°→75° to stop detecting ordinary course corrections as maneuvers.
Removed MAX_DELTA cap entirely — 180° crash-tacks are valid; the
circularMAD stability windows are the real quality gate.

TrackDetailSheet: dropped the arbitrary abs(delta)>70 Tack/Jibe
heuristic. Now uses true wind angle when available for classification;
falls back to "Maneuver" when TWD is absent.

SavedTracksFragment: added isLoadingTracks StateFlow (MainViewModel);
combine() suppresses the empty-state layout while the initial IO load
runs so it no longer flashes "No saved tracks yet" before data arrives.

16/16 TackDetector tests pass.

https://claude.ai/code/session_01KXYBuAHZkvv63DeUG6XaZD

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