feat(car): implement PR #5162 recommendations + official docs alignment

- Migrate to SendMessageUseCase (replaces raw commandSender.sendData())
- Migrate Messages tab to ConversationItem API (official required pattern)
- Add ConversationCallback for native reply + mark-as-read
- Add Status tab with device metrics (battery, utilization, uptime, TX/RX)
- Add ConversationShortcutManager + PersonIconFactory for notification linking
- Add CarScreenDataBuilder with pure testable functions (533-line test suite)
- Fix notification actions (SEMANTIC_ACTION_REPLY/MARK_AS_READ, setShowsUserInterface)
- Wire CarReplyReceiver to actually send messages and clear unreads
- Lower minCarApiLevel from 8 to 7 with graceful API 8 fallbacks
- Add android:permission on CarAppService for security
- Remove dead code: ConversationScreen, CarTtsEngine, messagesCache

Files added: ConversationShortcutManager, PersonIconFactory, CarScreenDataBuilder,
  ic_car_status.xml, CarScreenDataBuilderTest (533 lines)
Files removed: ConversationScreen.kt, CarTtsEngine.kt

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jamesarich

feature/car/src/main/AndroidManifest.xml

@@ -4,7 +4,8 @@
     <application>
         <service
             android:name="org.meshtastic.feature.car.service.MeshtasticCarAppService"
-            android:exported="true">
+            android:exported="true"
+            android:permission="androidx.car.app.CarAppService">
             <intent-filter>
                 <action android:name="androidx.car.app.CarAppService" />
                 <category android:name="androidx.car.app.category.MESSAGING" />
@@ -17,6 +18,6 @@
 
         <meta-data
             android:name="androidx.car.app.minCarApiLevel"
-            android:value="8" />
+            android:value="7" />
     </application>
 </manifest>

feature/car/src/main/kotlin/org/meshtastic/feature/car/model/CarUiModels.kt

@@ -74,3 +74,15 @@ data class EmergencyAlert(
     val timestamp: Long,
     val isActive: Boolean,
 )
+
+data class CarLocalStats(
+    val batteryLevel: Int = 0,
+    val hasBattery: Boolean = false,
+    val channelUtilization: Float = 0f,
+    val airUtilization: Float = 0f,
+    val totalNodes: Int = 0,
+    val onlineNodes: Int = 0,
+    val uptimeSeconds: Int = 0,
+    val numPacketsTx: Int = 0,
+    val numPacketsRx: Int = 0,
+)