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/markdownStyles.ts

@@ -0,0 +1,99 @@
+import { useThemeColor } from '@/hooks/useThemeColor';
+
+type ThemeColors = ReturnType<typeof useThemeColor>;
+
+/** Style map for the Markdown renderer, derived from the active theme colours. */
+export function createMarkdownStyles(colors: ThemeColors) {
+    return {
+        body: {
+            fontSize: 15,
+            lineHeight: 22,
+            color: colors.messageAssistantText,
+        },
+        strong: {
+            fontWeight: '700',
+            color: colors.text,
+        },
+        em: {
+            fontStyle: 'italic',
+        },
+        code_inline: {
+            backgroundColor: colors.backgroundColor,
+            color: colors.text,
+            paddingHorizontal: 5,
+            paddingVertical: 1,
+            borderRadius: 4,
+            fontSize: 13,
+            fontFamily: 'monospace',
+        },
+        fence: {
+            backgroundColor: colors.inputBg,
+            borderColor: colors.border,
+            borderWidth: 1,
+            color: colors.text,
+            padding: 12,
+            borderRadius: 8,
+            fontSize: 13,
+            fontFamily: 'monospace',
+            marginVertical: 8,
+            overflow: 'hidden',
+        },
+        code_block: {
+            backgroundColor: colors.inputBg,
+            color: colors.text,
+            padding: 12,
+            borderRadius: 8,
+            fontSize: 13,
+            fontFamily: 'monospace',
+            marginVertical: 8,
+        },
+        blockquote: {
+            borderLeftWidth: 3,
+            borderLeftColor: colors.border,
+            paddingLeft: 12,
+            marginVertical: 4,
+            color: colors.textMuted,
+        },
+        bullet_list: {
+            marginVertical: 4,
+        },
+        ordered_list: {
+            marginVertical: 4,
+        },
+        list_item: {
+            marginVertical: 2,
+            color: colors.messageAssistantText,
+        },
+        heading1: {
+            fontSize: 22,
+            fontWeight: '700',
+            marginVertical: 8,
+            color: colors.text,
+        },
+        heading2: {
+            fontSize: 19,
+            fontWeight: '700',
+            marginVertical: 6,
+            color: colors.text,
+        },
+        heading3: {
+            fontSize: 17,
+            fontWeight: '600',
+            marginVertical: 4,
+            color: colors.text,
+        },
+        link: {
+            color: '#3B82F6',
+            textDecorationLine: 'underline',
+        },
+        hr: {
+            backgroundColor: colors.border,
+            height: 1,
+            marginVertical: 12,
+        },
+        paragraph: {
+            marginVertical: 2,
+        },
+        // The Markdown renderer's style types do not accept every RN style key literal.
+    } as any;
+}