test: add UI smoke test for MainActivity placeholder

createAndroidComposeRule<MainActivity>() launches the activity in the
test process and verifies both app_name and app_tagline texts render
on the placeholder screen. Uses JUnit 4 + AndroidJUnit4 runner -
instrumented tests on Android still don't support JUnit Jupiter.

Test compiles (assembleDebugAndroidTest passes); execution requires
an emulator/device, performed in CI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/androidTest/java/de/jeanlucmakiola/calendula/MainActivitySmokeTest.kt

@@ -0,0 +1,26 @@
+package de.jeanlucmakiola.calendula
+
+import androidx.compose.ui.test.assertIsDisplayed
+import androidx.compose.ui.test.junit4.createAndroidComposeRule
+import androidx.compose.ui.test.onNodeWithText
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+class MainActivitySmokeTest {
+
+    @get:Rule
+    val composeTestRule = createAndroidComposeRule<MainActivity>()
+
+    @Test
+    fun appName_isDisplayed_onLaunch() {
+        composeTestRule.onNodeWithText("Calendula").assertIsDisplayed()
+    }
+
+    @Test
+    fun tagline_isDisplayed_onLaunch() {
+        composeTestRule.onNodeWithText("A modern calendar.").assertIsDisplayed()
+    }
+}