fix(location): persist pin across restarts + prevent Stop crash before simulation

[StephenDev0]

Fixes #346

Summary

Two bugs are addressed in LocationSimulationView (MapSelectionView.swift):

Bug 1 – Pin disappears after restarting the app

coordinate was a pure @State value that was never written to disk. On every launch it started as nil.

Fix: Added saveCoordinate() / loadCoordinate() helpers that persist the coordinate to UserDefaults (hasPinnedLocation, pinnedLocationLat, pinnedLocationLon). saveCoordinate() is called via a didSet observer on the state property; loadCoordinate() is called in .onAppear alongside loadBookmarks(). When the pin is cleared successfully the stored flag is set to false so the map starts clean next time.

Bug 2 – App crashes when tapping "Stop" immediately after dropping a pin

The "Stop" button was shown as soon as coordinate != nil, including before "Simulate Location" had ever been tapped. Pressing it called clear_simulated_location() on C global state that had never been initialised, causing a crash.

Fix: Added an isSimulating: Bool state flag.

• isSimulating is set to true only after simulate_location() returns 0 (success).
• The "Stop" button is now conditionally shown (if isSimulating) — it cannot be reached before a successful simulation.
• The "Simulate Location" button is disabled while isSimulating == true to prevent duplicate active sessions.
• isSimulating is reset to false after a successful clear_simulated_location() call.

Testing

1. Drop a pin → force-quit the app → reopen → pin should still be visible.
2. Drop a pin without tapping "Simulate Location" → "Stop" button should not appear.
3. Drop a pin → tap "Simulate Location" → tap "Stop" → should work without crashing.