feat: add sliding window trip generation for By Games mode

Adds a trip duration stepper (2-21 days) to the By Games planning mode.
When users select specific games, the planner now generates ALL possible
N-day windows containing those games (e.g., 7-day trips starting on
different days), finding additional games in each window to maximize
route options.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Trip/ViewModels/TripCreationViewModel.swift

@@ -137,6 +137,9 @@ final class TripCreationViewModel {
     var followTeamId: UUID?
     var useHomeLocation: Bool = true
 
+    // Game First Mode - Trip duration for sliding windows
+    var gameFirstTripDuration: Int = 7
+
     // MARK: - Dependencies
 
     private let planningEngine = TripPlanningEngine()
@@ -407,7 +410,8 @@ final class TripCreationViewModel {
             allowRepeatCities: allowRepeatCities,
             selectedRegions: selectedRegions,
             followTeamId: followTeamId,
-            useHomeLocation: useHomeLocation
+            useHomeLocation: useHomeLocation,
+            gameFirstTripDuration: gameFirstTripDuration
         )
 
         // Build planning request
@@ -589,7 +593,10 @@ final class TripCreationViewModel {
             numberOfDrivers: numberOfDrivers,
             maxDrivingHoursPerDriver: maxDrivingHoursPerDriver,
             allowRepeatCities: allowRepeatCities,
-            selectedRegions: selectedRegions
+            selectedRegions: selectedRegions,
+            followTeamId: followTeamId,
+            useHomeLocation: useHomeLocation,
+            gameFirstTripDuration: gameFirstTripDuration
         )
         return convertToTrip(option: option, preferences: preferences)
     }