refactor(settings): split SettingsScreen into section components and pure helpers

SettingsScreen CC 23 -> below 10. Form parsing, change detection and the dark-theme check moved to utils/ with unit tests; the dark switch now reads the OS scheme instead of comparing the background colour. Adds characterization tests for the screen.

components/settings/LabeledInput.tsx

@@ -0,0 +1,44 @@
+import { useThemeColor } from '@/hooks/useThemeColor';
+import React from 'react';
+import { StyleSheet, Text, TextInput, TextInputProps, View } from 'react-native';
+
+interface LabeledInputProps extends TextInputProps {
+    label: string;
+}
+
+export function LabeledInput({ label, style, ...inputProps }: LabeledInputProps) {
+    const colors = useThemeColor();
+
+    return (
+        <View style={styles.inputGroup}>
+            <Text style={[styles.label, { color: colors.textMuted }]}>{label}</Text>
+            <TextInput
+                style={[
+                    styles.textInput,
+                    { backgroundColor: colors.inputBg, borderColor: colors.inputBorder, color: colors.text },
+                    style,
+                ]}
+                placeholderTextColor={colors.textMuted}
+                {...inputProps}
+            />
+        </View>
+    );
+}
+
+const styles = StyleSheet.create({
+    inputGroup: {
+        marginBottom: 16,
+    },
+    label: {
+        fontSize: 14,
+        fontWeight: '500',
+        marginBottom: 8,
+    },
+    textInput: {
+        borderWidth: 1,
+        borderRadius: 12,
+        paddingHorizontal: 16,
+        paddingVertical: 12,
+        fontSize: 16,
+    },
+});