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/Core/Models/Domain/TripPreferences.swift

@@ -240,6 +240,9 @@ struct TripPreferences: Codable, Hashable {
     /// Whether to start/end from a home location (vs fly-in/fly-out)
     var useHomeLocation: Bool
 
+    /// Trip duration for gameFirst mode sliding windows (number of days)
+    var gameFirstTripDuration: Int
+
     init(
         planningMode: PlanningMode = .dateRange,
         startLocation: LocationInput? = nil,
@@ -262,7 +265,8 @@ struct TripPreferences: Codable, Hashable {
         allowRepeatCities: Bool = true,
         selectedRegions: Set<Region> = [.east, .central, .west],
         followTeamId: UUID? = nil,
-        useHomeLocation: Bool = true
+        useHomeLocation: Bool = true,
+        gameFirstTripDuration: Int = 7
     ) {
         self.planningMode = planningMode
         self.startLocation = startLocation
@@ -286,6 +290,7 @@ struct TripPreferences: Codable, Hashable {
         self.selectedRegions = selectedRegions
         self.followTeamId = followTeamId
         self.useHomeLocation = useHomeLocation
+        self.gameFirstTripDuration = gameFirstTripDuration
     }
 
     var totalDriverHoursPerDay: Double {