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.

utils/theme.ts

@@ -0,0 +1,7 @@
+/** Whether the dark theme is in effect for a stored preference and the OS scheme. */
+export function isDarkTheme(
+    preference: 'light' | 'dark' | 'system',
+    systemScheme: 'light' | 'dark',
+): boolean {
+    return preference === 'dark' || (preference === 'system' && systemScheme === 'dark');
+}