feat: Implement Pareto-front ranking integration with multi-criteria sorting

- Add RankingMode field to SearchOptions (fastest/fewest_transfers/cheapest)
- Update FindRoutesPareto to respect ranking mode when sorting
- Add ranking_mode query parameter to RouteSearch endpoint
- Add TestParetoFrontGeneration with subtests for all three modes

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

cmd/api/handlers.go

@@ -173,6 +173,9 @@ func RouteSearch(hc *HandlerContext, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Read ranking mode from query parameters (for UI controls)
+	rankingMode := r.URL.Query().Get("ranking_mode")
+
 	// Build query parameters for route search
 	// Use city codes as origin/destination identifiers
 	// In a full implementation, this would use Yandex /search, but for now
@@ -181,6 +184,8 @@ func RouteSearch(hc *HandlerContext, w http.ResponseWriter, r *http.Request) {
 	// Create search options with default max transfers
 	opts := routing.SearchOptions{
 		MaxTransfers: 5,
+		// Set ranking mode from UI query parameter if provided
+		RankingMode: rankingMode,
 	}
 
 	// Run Pareto-optimal route search using the graph