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/Planning/Engine/ScenarioBPlanner.swift

@@ -221,13 +221,19 @@ final class ScenarioBPlanner: ScenarioPlanner {
         request: PlanningRequest
     ) -> [DateInterval] {
 
-        // If explicit date range exists, use it
-        if let dateRange = request.dateRange {
+        // For gameFirst mode, ALWAYS use sliding windows with gameFirstTripDuration
+        // This generates all possible N-day windows containing the selected games
+        let isGameFirstMode = request.preferences.planningMode == .gameFirst
+        let duration = isGameFirstMode
+            ? request.preferences.gameFirstTripDuration
+            : request.preferences.effectiveTripDuration
+
+        // If not gameFirst and explicit date range exists, use it directly
+        if !isGameFirstMode, let dateRange = request.dateRange {
             return [dateRange]
         }
 
-        // Otherwise, use trip duration to create sliding windows
-        let duration = request.preferences.effectiveTripDuration
+        // Use trip duration to create sliding windows
         guard duration > 0 else { return [] }
 
         // Find the span of selected games