refactor(chat): split MessageBubble into avatar, content and code block components

MessageBubble CC 19 -> below 10. The fence and indented code block rules were identical and are now one CodeBlock component; the Markdown style map lives in markdownStyles.ts. Adds characterization tests for the bubble (roles, actions visibility, edited badge, copy feedback).

components/chat/CodeBlock.tsx

@@ -0,0 +1,73 @@
+import { Check, Copy } from 'lucide-react-native';
+import React from 'react';
+import { ScrollView, StyleSheet, Text, TouchableOpacity, View } from 'react-native';
+
+interface CodeBlockProps {
+    language: string;
+    content: string;
+    copied: boolean;
+    onCopy: (text: string) => void;
+}
+
+export function CodeBlock({ language, content, copied, onCopy }: CodeBlockProps) {
+    const CopyIcon = copied ? Check : Copy;
+
+    return (
+        <View style={styles.codeBlockWrapper}>
+            <View style={styles.codeBlockHeader}>
+                <Text style={styles.codeBlockLanguage}>{language}</Text>
+                <TouchableOpacity onPress={() => onCopy(content)} style={styles.copyButton}>
+                    <CopyIcon size={14} color="#A0A0A0" />
+                    <Text style={styles.copyText}>{copied ? 'Copied!' : 'Copy'}</Text>
+                </TouchableOpacity>
+            </View>
+            <ScrollView horizontal style={styles.codeScrollView} showsHorizontalScrollIndicator={false}>
+                <Text style={styles.codeText}>{content}</Text>
+            </ScrollView>
+        </View>
+    );
+}
+
+const styles = StyleSheet.create({
+    codeBlockWrapper: {
+        backgroundColor: '#1E1E1E',
+        borderRadius: 8,
+        marginVertical: 8,
+        overflow: 'hidden',
+        width: '100%',
+    },
+    codeBlockHeader: {
+        flexDirection: 'row',
+        justifyContent: 'space-between',
+        alignItems: 'center',
+        backgroundColor: '#2D2D2D',
+        paddingHorizontal: 12,
+        paddingVertical: 8,
+        borderBottomWidth: 1,
+        borderBottomColor: '#404040',
+    },
+    codeBlockLanguage: {
+        color: '#A0A0A0',
+        fontSize: 12,
+        fontFamily: 'monospace',
+        textTransform: 'lowercase',
+    },
+    copyButton: {
+        flexDirection: 'row',
+        alignItems: 'center',
+        gap: 4,
+        padding: 4,
+    },
+    copyText: {
+        color: '#A0A0A0',
+        fontSize: 12,
+    },
+    codeScrollView: {
+        padding: 12,
+    },
+    codeText: {
+        color: '#D4D4D4',
+        fontSize: 13,
+        fontFamily: 'monospace',
+    },
+});