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.

__tests__/SettingsScreen.test.tsx

@@ -0,0 +1,172 @@
+import { act, fireEvent, render, screen } from '@testing-library/react-native';
+import React from 'react';
+import SettingsScreen from '../app/settings/index';
+import * as api from '../services/lmStudioApi';
+import { DEFAULT_SERVER_CONFIG } from '../services/types';
+import { useAppStore } from '../store/useAppStore';
+
+jest.mock('../services/lmStudioApi');
+
+let mockSystemScheme: 'light' | 'dark' = 'light';
+jest.mock('../hooks/useColorScheme', () => ({
+    useColorScheme: () => mockSystemScheme,
+}));
+
+const mockApi = api as jest.Mocked<typeof api>;
+const state = () => useAppStore.getState();
+
+const hostInput = () => screen.getByPlaceholderText('localhost');
+const portInput = () => screen.getByPlaceholderText('1234');
+const keyInput = () => screen.getByPlaceholderText('Leave empty for local');
+
+beforeEach(() => {
+    jest.resetAllMocks();
+    mockSystemScheme = 'light';
+    useAppStore.setState(useAppStore.getInitialState(), true);
+    mockApi.getModels.mockResolvedValue([]);
+});
+
+describe('SettingsScreen connection form', () => {
+    it('is filled from the stored configuration and hides "Save Changes" until something changes', () => {
+        useAppStore.setState({ serverConfig: { host: '192.168.0.5', port: 4321, apiKey: 'k' } });
+        render(<SettingsScreen />);
+
+        expect(hostInput().props.value).toBe('192.168.0.5');
+        expect(portInput().props.value).toBe('4321');
+        expect(keyInput().props.value).toBe('k');
+        expect(screen.queryByText('Save Changes')).toBeNull();
+    });
+
+    it('shows "Save Changes" for each changed field', () => {
+        render(<SettingsScreen />);
+
+        fireEvent.changeText(hostInput(), 'other');
+        expect(screen.getByText('Save Changes')).toBeTruthy();
+        fireEvent.changeText(hostInput(), DEFAULT_SERVER_CONFIG.host);
+        expect(screen.queryByText('Save Changes')).toBeNull();
+
+        fireEvent.changeText(portInput(), '999');
+        expect(screen.getByText('Save Changes')).toBeTruthy();
+        fireEvent.changeText(portInput(), String(DEFAULT_SERVER_CONFIG.port));
+        expect(screen.queryByText('Save Changes')).toBeNull();
+
+        fireEvent.changeText(keyInput(), 'secret');
+        expect(screen.getByText('Save Changes')).toBeTruthy();
+    });
+
+    it('saves trimmed values', () => {
+        render(<SettingsScreen />);
+        fireEvent.changeText(hostInput(), '  192.168.0.9  ');
+        fireEvent.changeText(portInput(), '8080');
+        fireEvent.changeText(keyInput(), '  key  ');
+        fireEvent.press(screen.getByText('Save Changes'));
+
+        expect(state().serverConfig).toEqual({ host: '192.168.0.9', port: 8080, apiKey: 'key' });
+    });
+
+    it('falls back to localhost:1234 and no API key when the fields are blank or invalid', () => {
+        useAppStore.setState({ serverConfig: { host: 'lan', port: 4321, apiKey: 'k' } });
+        render(<SettingsScreen />);
+        fireEvent.changeText(hostInput(), '   ');
+        fireEvent.changeText(portInput(), 'abc');
+        fireEvent.changeText(keyInput(), '  ');
+        fireEvent.press(screen.getByText('Save Changes'));
+
+        expect(state().serverConfig).toEqual({ host: 'localhost', port: 1234, apiKey: undefined });
+    });
+});
+
+describe('SettingsScreen connection test', () => {
+    it('saves the configuration, reports success and loads the models', async () => {
+        mockApi.testConnection.mockResolvedValue(true);
+        render(<SettingsScreen />);
+        fireEvent.changeText(hostInput(), '10.0.0.2');
+
+        await act(async () => {
+            fireEvent.press(screen.getByText('Test Connection'));
+        });
+
+        expect(mockApi.testConnection).toHaveBeenCalledWith({ host: '10.0.0.2', port: 1234, apiKey: undefined });
+        expect(state().serverConfig.host).toBe('10.0.0.2');
+        expect(state().isConnected).toBe(true);
+        expect(mockApi.getModels).toHaveBeenCalledTimes(1);
+        expect(screen.getByText('Connection successful. API is ready.')).toBeTruthy();
+        expect(screen.getByText('Connected')).toBeTruthy();
+    });
+
+    it('reports failure and does not load models', async () => {
+        mockApi.testConnection.mockResolvedValue(false);
+        render(<SettingsScreen />);
+
+        await act(async () => {
+            fireEvent.press(screen.getByText('Test Connection'));
+        });
+
+        expect(state().isConnected).toBe(false);
+        expect(mockApi.getModels).not.toHaveBeenCalled();
+        expect(screen.getByText('Connection failed. Verify host and port.')).toBeTruthy();
+        expect(screen.getByText('Disconnected')).toBeTruthy();
+    });
+
+    it('shows a spinner and ignores presses while the test is running', async () => {
+        let resolveTest: (ok: boolean) => void = () => {};
+        mockApi.testConnection.mockReturnValue(new Promise((resolve) => (resolveTest = resolve)));
+        render(<SettingsScreen />);
+
+        await act(async () => {
+            fireEvent.press(screen.getByText('Test Connection'));
+        });
+        expect(screen.queryByText('Test Connection')).toBeNull();
+
+        await act(async () => {
+            resolveTest(false);
+        });
+        expect(screen.getByText('Test Connection')).toBeTruthy();
+    });
+
+    it('clears a previous result when the configuration is saved again', async () => {
+        mockApi.testConnection.mockResolvedValue(false);
+        render(<SettingsScreen />);
+        await act(async () => {
+            fireEvent.press(screen.getByText('Test Connection'));
+        });
+        expect(screen.getByText(/Connection failed/)).toBeTruthy();
+
+        fireEvent.changeText(hostInput(), 'changed');
+        fireEvent.press(screen.getByText('Save Changes'));
+        expect(screen.queryByText(/Connection failed/)).toBeNull();
+    });
+});
+
+describe('SettingsScreen dark mode switch', () => {
+    const darkSwitch = () => screen.UNSAFE_getByType(require('react-native').Switch);
+
+    it('is on for the dark preference and switches to light', () => {
+        useAppStore.setState({ theme: 'dark' });
+        render(<SettingsScreen />);
+        expect(darkSwitch().props.value).toBe(true);
+
+        fireEvent(darkSwitch(), 'valueChange', false);
+        expect(state().theme).toBe('light');
+    });
+
+    it('is off for the light preference and switches to dark', () => {
+        useAppStore.setState({ theme: 'light' });
+        render(<SettingsScreen />);
+        expect(darkSwitch().props.value).toBe(false);
+
+        fireEvent(darkSwitch(), 'valueChange', true);
+        expect(state().theme).toBe('dark');
+    });
+
+    it('follows the system scheme when the preference is "system"', () => {
+        mockSystemScheme = 'dark';
+        const first = render(<SettingsScreen />);
+        expect(darkSwitch().props.value).toBe(true);
+        first.unmount();
+
+        mockSystemScheme = 'light';
+        render(<SettingsScreen />);
+        expect(darkSwitch().props.value).toBe(false);
+    });
+});