feat: magnifier view setup + unit тесты для TextSelectionHelper

- setMagnifierView(view) в ChatDetailScreen через LaunchedEffect
- 9 unit тестов: initial state, clear, getSelectedText, boundary checks

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/java/com/rosetta/messenger/ui/chats/ChatDetailScreen.kt

@@ -396,6 +396,7 @@ fun ChatDetailScreen(
 
         // 🔤 TEXT SELECTION - Telegram-style character-level selection
         val textSelectionHelper = remember { com.rosetta.messenger.ui.chats.components.TextSelectionHelper() }
+        LaunchedEffect(Unit) { textSelectionHelper.setMagnifierView(view) }
 
         // 💬 MESSAGE CONTEXT MENU STATE
         var contextMenuMessage by remember { mutableStateOf<ChatMessage?>(null) }