B1 (#173): tell the rail from the bottom bar, and the Convert tab from the Join tab

Two assertion gaps, not coverage gaps, which is why they lasted. AppRootRestorationTest
already drives AppRoot at Compact and Expanded, so JaCoCo is green on useRail -- but it
asserts only that the selected tab survives recreation, through a stub `content`
composable. Nothing anywhere queried for a rail or a bar, and nothing composed the real
screens. Measured before this file existed:

  - transposing the NavigationRail and NavigationBar bodies passed the entire suite
  - transposing Content's two arms passed it too

A tablet showing phone chrome, or the Convert tab opening the Join screen, and 546 tests
with nothing to say about either. AppRoot's own KDoc is why that matters more than it
looks: from targetSdk 37 the app is resized and rotated whether or not it is ready, so the
width class is not a preference.

WindowWidthSizeClass.Medium appears in no test in either source set today. useRail is
`!= Compact`, so Medium takes the rail; narrowing it to `== Expanded` is one character and
breaks every tablet and unfolded foldable. That mutation is red now, and it is red only
because of the Medium test -- the Compact and Expanded ones both survive it.

Two things this needed:

**createAndroidComposeRule rather than createComposeRule.** Rendering AppRoot with its
default content reaches ConverterScreen's `viewModel = viewModel()`, which needs a
ViewModelStoreOwner. It works because both ViewModels are
`@JvmOverloads constructor(app: Application, ...)` so AndroidViewModelFactory can build
them, and because ui-test-manifest's debugImplementation entry already puts a
ComponentActivity in the merged manifest the unit tests build against -- which
app/build.gradle.kts says in terms. Checked with a throwaway spike before the ticket was
filed, rather than discovered here.

**Two tags, applied inside main.** The only production change: TestTags.Shell, set on the
rail and the bar. There is no other way to tell the two apart -- both render the same two
destinations with the same labels and the same selection state, so any assertion writable
without them is satisfied by either layout. In TestTags and applied by the shell rather
than handed down by the test, for the reason that file's KDoc gives: a tag the test
supplies proves only that the test set it. TagTableUniquenessTest covers the new group.

564 -> 568 JVM tests, 0 failures.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/main/java/org/libremediaconverter/MainActivity.kt

@@ -24,9 +24,11 @@ import androidx.compose.runtime.saveable.Saver
 import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.testTag
 import androidx.media3.common.util.UnstableApi
 import org.libremediaconverter.convert.ConverterScreen
 import org.libremediaconverter.join.JoinScreen
+import org.libremediaconverter.ui.TestTags
 import org.libremediaconverter.ui.theme.LibreMediaConverterTheme
 
 /**
@@ -114,7 +116,7 @@ internal fun AppRoot(
 
     if (useRail) {
         Row(modifier = Modifier.fillMaxSize()) {
-            NavigationRail {
+            NavigationRail(modifier = Modifier.testTag(TestTags.Shell.NAVIGATION_RAIL)) {
                 Destination.entries.forEach { item ->
                     NavigationRailItem(
                         selected = destination == item,
@@ -132,7 +134,7 @@ internal fun AppRoot(
         Scaffold(
             modifier = Modifier.fillMaxSize(),
             bottomBar = {
-                NavigationBar {
+                NavigationBar(modifier = Modifier.testTag(TestTags.Shell.NAVIGATION_BAR)) {
                     Destination.entries.forEach { item ->
                         NavigationBarItem(
                             selected = destination == item,

app/src/main/java/org/libremediaconverter/ui/TestTags.kt

@@ -56,6 +56,20 @@ object TestTags {
      */
     const val RETRY_SAVE: String = "action.retrySave"
 
+    /**
+     * The adaptive shell around both screens -- `AppRoot`'s two layouts.
+     *
+     * Named because there is no other way to tell them apart from a test. Both render the same two
+     * destinations with the same labels and the same selection state, so every assertion that could
+     * be written without these tags is satisfied by either layout, and transposing the two bodies
+     * passed the whole suite. Exactly one of the two exists at a time, which is what makes
+     * `assertExists` / `assertDoesNotExist` on this pair a statement about the width class.
+     */
+    object Shell {
+        const val NAVIGATION_RAIL: String = "shell.navigationRail"
+        const val NAVIGATION_BAR: String = "shell.navigationBar"
+    }
+
     /** `ConverterScreen`. */
     object Converter {
         const val CHOOSE_FILE: String = "converter.chooseFile"