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/DashboardView.swift

@@ -120,6 +120,7 @@ struct DashboardView: View {
         ScrollView {
             VStack(alignment: .leading, spacing: contentSpacing) {
                 headerSection
+                    .platformFocusSection()
 
                 if viewModel.isLoading {
                     loadingState
@@ -127,7 +128,9 @@ struct DashboardView: View {
                     errorState(error)
                 } else {
                     overviewStrip
+                        .platformFocusSection()
                     heroAndControlSection
+                        .platformFocusSection()
 
                     if !viewModel.liveGames.isEmpty {
                         gameShelf(