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/serverConfig.ts

@@ -0,0 +1,25 @@
+import { DEFAULT_SERVER_CONFIG, ServerConfig } from '../services/types';
+
+export interface ServerConfigForm {
+    host: string;
+    port: string;
+    apiKey: string;
+}
+
+/** Turns the raw text fields of the settings form into a usable ServerConfig. */
+export function buildServerConfig(form: ServerConfigForm): ServerConfig {
+    return {
+        host: form.host.trim() || DEFAULT_SERVER_CONFIG.host,
+        port: parseInt(form.port, 10) || DEFAULT_SERVER_CONFIG.port,
+        apiKey: form.apiKey.trim() || undefined,
+    };
+}
+
+/** True when the form differs from what is currently stored. */
+export function isConfigChanged(form: ServerConfigForm, saved: ServerConfig): boolean {
+    return (
+        form.host !== saved.host ||
+        form.port !== saved.port.toString() ||
+        form.apiKey !== (saved.apiKey ?? '')
+    );
+}