feat: implement MeshtasticNavDisplay and centralize Navigation 3 configuration

- Introduce `MeshtasticNavDisplay` in `core:ui` to encapsulate shared Navigation 3 configuration, including entry decorators, scene strategies, and transitions.
- Integrate `rememberViewModelStoreNavEntryDecorator` to provide entry-scoped `ViewModelStoreOwner` support, ensuring ViewModels are automatically cleared when their backstack entry is popped.
- Configure `DialogSceneStrategy` to enable navigation-driven dialogs that respect backstack lifecycle and predictive back gestures.
- Implement a standardized 350 ms crossfade transition for both forward and pop navigation across all platforms.
- Refactor `app` and `desktop` host modules to utilize `MeshtasticNavDisplay`, removing redundant manual configuration of `NavDisplay` and decorators.
- Update `core:ui` dependencies to include `lifecycle-viewmodel-navigation3` and move it away from platform-specific host modules.
- Update architectural documentation (`AGENTS.md`, `GEMINI.md`, and decision logs) to reflect the adoption of centralized navigation and ViewModel scoping patterns.

core/ui/build.gradle.kts

@@ -57,6 +57,8 @@ kotlin {
             implementation(libs.jetbrains.compose.material3.adaptive.navigation.suite)
             implementation(libs.jetbrains.navigationevent.compose)
             implementation(libs.jetbrains.navigation3.ui)
+            implementation(libs.jetbrains.lifecycle.viewmodel.navigation3)
+            implementation(libs.jetbrains.lifecycle.viewmodel.compose)
         }
 
         val jvmAndroidMain by getting { dependencies { implementation(libs.compose.multiplatform.ui.tooling) } }

core/ui/src/commonMain/kotlin/org/meshtastic/core/ui/component/MeshtasticNavDisplay.kt

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2025-2026 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.ui.component
+
+import androidx.compose.animation.AnimatedContentTransitionScope
+import androidx.compose.animation.ContentTransform
+import androidx.compose.animation.core.tween
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.lifecycle.viewmodel.navigation3.rememberViewModelStoreNavEntryDecorator
+import androidx.navigation3.runtime.NavEntry
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.rememberSaveableStateHolderNavEntryDecorator
+import androidx.navigation3.scene.DialogSceneStrategy
+import androidx.navigation3.scene.Scene
+import androidx.navigation3.scene.SinglePaneSceneStrategy
+import androidx.navigation3.ui.NavDisplay
+
+/**
+ * Duration in milliseconds for the shared crossfade transition between navigation scenes.
+ *
+ * This is faster than the library's Android default (700 ms) and matches Material 3 motion guidance for medium-emphasis
+ * container transforms (~300-400 ms).
+ */
+private const val TRANSITION_DURATION_MS = 350
+
+/**
+ * Shared [NavDisplay] wrapper that configures the standard Meshtastic entry decorators, scene strategies, and
+ * transition animations for all platform hosts.
+ *
+ * **Entry decorators** (applied to every backstack entry):
+ * - [rememberSaveableStateHolderNavEntryDecorator] — saveable state per entry.
+ * - [rememberViewModelStoreNavEntryDecorator] — entry-scoped `ViewModelStoreOwner` so that ViewModels obtained via
+ *   `koinViewModel()` are automatically cleared when the entry is popped.
+ *
+ * **Scene strategies** (evaluated in order):
+ * - [DialogSceneStrategy] — entries annotated with `metadata = DialogSceneStrategy.dialog()` render as overlay
+ *   [Dialog][androidx.compose.ui.window.Dialog] windows with proper backstack lifecycle.
+ * - [SinglePaneSceneStrategy] — default single-pane fallback.
+ *
+ * **Transitions**: A uniform 350 ms crossfade for both forward and pop navigation.
+ *
+ * @param backStack the navigation backstack, typically from [rememberNavBackStack].
+ * @param entryProvider the entry provider built from feature navigation graphs.
+ * @param modifier modifier applied to the underlying [NavDisplay].
+ */
+@Composable
+fun MeshtasticNavDisplay(
+    backStack: List<NavKey>,
+    entryProvider: (key: NavKey) -> NavEntry<NavKey>,
+    modifier: Modifier = Modifier,
+) {
+    NavDisplay(
+        backStack = backStack,
+        entryProvider = entryProvider,
+        entryDecorators =
+        listOf(rememberSaveableStateHolderNavEntryDecorator(), rememberViewModelStoreNavEntryDecorator()),
+        sceneStrategies = listOf(DialogSceneStrategy(), SinglePaneSceneStrategy()),
+        transitionSpec = meshtasticTransitionSpec(),
+        popTransitionSpec = meshtasticTransitionSpec(),
+        modifier = modifier,
+    )
+}
+
+/**
+ * Shared crossfade [ContentTransform] used for both forward and pop navigation. Returns a lambda compatible with
+ * [NavDisplay]'s `transitionSpec` / `popTransitionSpec` parameters.
+ */
+private fun meshtasticTransitionSpec(): AnimatedContentTransitionScope<Scene<NavKey>>.() -> ContentTransform = {
+    ContentTransform(
+        fadeIn(animationSpec = tween(TRANSITION_DURATION_MS)),
+        fadeOut(animationSpec = tween(TRANSITION_DURATION_MS)),
+    )
+}