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) }

app/src/test/java/com/rosetta/messenger/ui/chats/components/TextSelectionHelperTest.kt

@@ -0,0 +1,77 @@
+package com.rosetta.messenger.ui.chats.components
+
+import org.junit.Assert.*
+import org.junit.Before
+import org.junit.Test
+
+class TextSelectionHelperTest {
+
+    private lateinit var helper: TextSelectionHelper
+
+    @Before
+    fun setup() {
+        helper = TextSelectionHelper()
+    }
+
+    @Test
+    fun `initial state is not active`() {
+        assertFalse(helper.isActive)
+        assertFalse(helper.isInSelectionMode)
+        assertEquals(-1, helper.selectionStart)
+        assertEquals(-1, helper.selectionEnd)
+        assertNull(helper.selectedMessageId)
+    }
+
+    @Test
+    fun `clear resets all state`() {
+        helper.clear()
+        assertFalse(helper.isActive)
+        assertEquals(-1, helper.selectionStart)
+        assertEquals(-1, helper.selectionEnd)
+        assertNull(helper.selectedMessageId)
+        assertNull(helper.layoutInfo)
+        assertFalse(helper.showToolbar)
+        assertFalse(helper.movingHandle)
+    }
+
+    @Test
+    fun `getSelectedText returns null when not active`() {
+        assertNull(helper.getSelectedText())
+    }
+
+    @Test
+    fun `updateSelectionEnd does not change when not active`() {
+        helper.updateSelectionEnd(5)
+        assertEquals(-1, helper.selectionEnd)
+    }
+
+    @Test
+    fun `updateSelectionStart does not change when not active`() {
+        helper.updateSelectionStart(0)
+        assertEquals(-1, helper.selectionStart)
+    }
+
+    @Test
+    fun `getCharOffsetFromCoords returns -1 when no layout`() {
+        assertEquals(-1, helper.getCharOffsetFromCoords(100, 100))
+    }
+
+    @Test
+    fun `selectAll does nothing when no layout`() {
+        helper.selectAll()
+        assertEquals(-1, helper.selectionStart)
+        assertEquals(-1, helper.selectionEnd)
+    }
+
+    @Test
+    fun `moveHandle does nothing when not moving`() {
+        helper.moveHandle(100f, 100f)
+        assertFalse(helper.movingHandle)
+    }
+
+    @Test
+    fun `endHandleDrag sets movingHandle to false`() {
+        helper.endHandleDrag()
+        assertFalse(helper.movingHandle)
+    }
+}