test(US-14): add instrumented UI tests for PacerScreen

Tests cover: app launch, START/STOP toggle, preset BPM button,
and service running/stopped state. Run with:
./gradlew connectedAndroidTest

app/src/androidTest/kotlin/net/jeena/pacer/PacerScreenTest.kt

@@ -0,0 +1,68 @@
+package net.jeena.pacer
+
+import androidx.compose.ui.test.assertIsDisplayed
+import androidx.compose.ui.test.junit4.createAndroidComposeRule
+import androidx.compose.ui.test.onNodeWithText
+import androidx.compose.ui.test.performClick
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+
+/**
+ * Instrumented UI tests for PacerScreen (US-14).
+ * Run with: ./gradlew connectedAndroidTest
+ */
+@RunWith(AndroidJUnit4::class)
+class PacerScreenTest {
+
+    @get:Rule
+    val composeRule = createAndroidComposeRule<MainActivity>()
+
+    @Test
+    fun appLaunches_andShowsStartButton() {
+        composeRule.onNodeWithText("START").assertIsDisplayed()
+    }
+
+    @Test
+    fun appLaunches_andShowsTitle() {
+        composeRule.onNodeWithText("PACER").assertIsDisplayed()
+    }
+
+    @Test
+    fun tappingStart_changesButtonToStop() {
+        composeRule.onNodeWithText("START").performClick()
+        composeRule.onNodeWithText("STOP").assertIsDisplayed()
+    }
+
+    @Test
+    fun tappingStop_changesButtonBackToStart() {
+        composeRule.onNodeWithText("START").performClick()
+        composeRule.onNodeWithText("STOP").performClick()
+        composeRule.onNodeWithText("START").assertIsDisplayed()
+    }
+
+    @Test
+    fun presetButton_updatesBpmDisplay() {
+        // Tap the "100 / SLOW" preset
+        composeRule.onNodeWithText("100").performClick()
+        composeRule.onNodeWithText("100").assertIsDisplayed()
+    }
+
+    @Test
+    fun serviceRunning_afterTappingStart() {
+        composeRule.onNodeWithText("START").performClick()
+        composeRule.waitForIdle()
+        assert(PacerService.isRunning)
+        // Clean up
+        composeRule.onNodeWithText("STOP").performClick()
+    }
+
+    @Test
+    fun serviceStopped_afterTappingStop() {
+        composeRule.onNodeWithText("START").performClick()
+        composeRule.onNodeWithText("STOP").performClick()
+        composeRule.waitForIdle()
+        assert(!PacerService.isRunning)
+    }
+}