refactor: move selectMainTab logic to MutableList<Route> extension function

shared/src/commonMain/kotlin/bzh/ajaury/chombev/App.kt

@@ -47,15 +47,6 @@ fun App() {
             mutableStateListOf(Route.Records)
         }
 
-        // Selecting a main tab keeps Records as the root, so system Back always returns home.
-        fun selectMainTab(page: WebPage?) {
-            backStack.clear()
-            backStack.add(Route.Records)
-            if (page != null) {
-                backStack.add(Route.Web(page = page))
-            }
-        }
-
         val darkTheme = isSystemInDarkTheme()
 
         // The current top destination drives the shared chrome: the top and bottom bars are shown
@@ -77,9 +68,9 @@ fun App() {
                     if (isMainScreen) {
                         MainBottomBar(
                             selectedPage = selectedPage,
-                            onRecordsClick = { selectMainTab(null) },
-                            onDeepenClick = { selectMainTab(WebPages.DEEPEN) },
-                            onAboutClick = { selectMainTab(WebPages.ABOUT) },
+                            onRecordsClick = { backStack.selectMainTab(null) },
+                            onDeepenClick = { backStack.selectMainTab(WebPages.DEEPEN) },
+                            onAboutClick = { backStack.selectMainTab(WebPages.ABOUT) },
                         )
                     }
                 },
@@ -119,3 +110,12 @@ fun App() {
         }
     }
 }
+
+// Selecting a main tab keeps Records as the root, so system Back always returns home.
+private fun MutableList<Route>.selectMainTab(page: WebPage?) {
+    this.clear()
+    this.add(Route.Records)
+    if (page != null) {
+        this.add(Route.Web(page = page))
+    }
+}