feat: implement global SnackbarManager and consolidate common UI setup (#4909)

Signed-off-by: James Rich <2199651+jamesarich@users.noreply.github.com>

core/navigation/src/commonMain/kotlin/org/meshtastic/core/navigation/NavBackStackExt.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2026 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.meshtastic.core.navigation
+
+import androidx.navigation3.runtime.NavKey
+
+/**
+ * Replaces the current back stack with the given top-level route. Clears the back stack and sets the new route as the
+ * root destination.
+ */
+fun MutableList<NavKey>.navigateTopLevel(route: NavKey) {
+    if (isNotEmpty()) {
+        this[0] = route
+        while (size > 1) {
+            removeAt(lastIndex)
+        }
+    } else {
+        add(route)
+    }
+}