fix: Show all system items (quit, help, theme) and fix deduplication

- StatusBarState now initializes with all 3 system items
- Added deduplication logic to StatusBar.renderToBuffer
- User items with same shortcut override system items
- Updated Example App to not duplicate 'q quit'
- Updated tests for 3 system items instead of 1

Sources/TUIKitExample/ContentView.swift

@@ -45,7 +45,6 @@ struct ContentView: View {
                     StatusBarItem(shortcut: Shortcut.arrowsUpDown, label: "nav")
                     StatusBarItem(shortcut: Shortcut.enter, label: "select", key: .enter)
                     StatusBarItem(shortcut: Shortcut.range("1", "6"), label: "jump")
-                    StatusBarItem(shortcut: Shortcut.quit, label: "quit")
                 }
         case .textStyles:
             TextStylesPage()
@@ -74,8 +73,7 @@ struct ContentView: View {
             StatusBarItem(shortcut: Shortcut.escape, label: "back") {
                 ExampleAppState.shared.currentPage = .menu
             },
-            StatusBarItem(shortcut: Shortcut.arrowsUpDown, label: "scroll"),
-            StatusBarItem(shortcut: Shortcut.quit, label: "quit")
+            StatusBarItem(shortcut: Shortcut.arrowsUpDown, label: "scroll")
         ]
     }
 }