Fix tvOS focus navigation: add focusSection to all major UI sections

Focus couldn't move from DashboardView controls up to the
CategoryPillBar, or between sections within screens. Added
.platformFocusSection() to:

- ContentView: pill bar and content area as separate focus sections
- DashboardView: header, overview strip, hero+control section
- LeagueCenterView: schedule, standings, leaders, teams sections

This lets the tvOS Focus Engine navigate vertically between all
sections with d-pad up/down.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mlbTVOS/Views/LeagueCenterView.swift

@@ -35,16 +35,18 @@ struct LeagueCenterView: View {
                 }
 
                 scheduleSection
+                    .platformFocusSection()
 
                 #if os(tvOS)
-                // Side-by-side: standings left, leaders right
                 HStack(alignment: .top, spacing: 24) {
                     standingsSection
                         .frame(maxWidth: .infinity)
+                        .platformFocusSection()
 
                     if !viewModel.leagueLeaders.isEmpty {
                         leadersColumnSection
                             .frame(width: 420)
+                            .platformFocusSection()
                     }
                 }
                 #else
@@ -56,6 +58,7 @@ struct LeagueCenterView: View {
                 #endif
 
                 teamsSection
+                    .platformFocusSection()
 
                 if let selectedTeam = viewModel.selectedTeam {
                     teamProfileSection(team: selectedTeam)