refactor(chat): split ChatScreen and reuse regenerateResponse after editing a message

The edit path duplicated the streaming flow with inline require() calls; it now reuses regenerateResponse, which also fixes:
- edited conversations were never saved to History
- the answer generated after an edit could not be stopped (no abort handle kept)
Stopping a stream now saves the partial conversation, like finishing or failing does.
Adds characterization tests for ChatScreen and useLMStudio.

components/chat/OfflineBanner.tsx

@@ -0,0 +1,31 @@
+import { useThemeColor } from '@/hooks/useThemeColor';
+import React from 'react';
+import { StyleSheet, Text, View } from 'react-native';
+
+export function OfflineBanner() {
+    const colors = useThemeColor();
+
+    return (
+        <View
+            accessibilityRole="alert"
+            style={[styles.banner, { backgroundColor: colors.bannerBg, borderBottomColor: colors.bannerBg }]}
+        >
+            <Text style={[styles.bannerText, { color: colors.bannerText }]}>
+                ⚠️ Server offline. Check Settings → Server Connection.
+            </Text>
+        </View>
+    );
+}
+
+const styles = StyleSheet.create({
+    banner: {
+        paddingHorizontal: 16,
+        paddingVertical: 10,
+        borderBottomWidth: 1,
+    },
+    bannerText: {
+        fontSize: 13,
+        textAlign: 'center',
+        fontWeight: '500',
+    },
+});