fix: add persisted theme toggle

package.json

@@ -4,7 +4,7 @@
   "description": "A simple CSV parser - BountyPay workflow demo",
   "main": "src/parser.js",
   "scripts": {
-    "test": "node test/parser.test.js"
+    "test": "node test/parser.test.js && node test/theme-preference.test.js"
   },
   "license": "MIT"
 }

src/theme-preference.js

@@ -0,0 +1,63 @@
+const THEME_KEY = 'theme';
+const LIGHT_THEME = 'light';
+const DARK_THEME = 'dark';
+
+function normalizeTheme(theme) {
+  return theme === DARK_THEME ? DARK_THEME : LIGHT_THEME;
+}
+
+function createThemeToggleController({
+  storage,
+  applyTheme,
+  updateToggle,
+  initialTheme,
+  storageKey = THEME_KEY,
+}) {
+  if (!storage || typeof storage.getItem !== 'function' || typeof storage.setItem !== 'function') {
+    throw new TypeError('storage must provide getItem and setItem');
+  }
+  if (typeof applyTheme !== 'function') {
+    throw new TypeError('applyTheme must be a function');
+  }
+  if (typeof updateToggle !== 'function') {
+    throw new TypeError('updateToggle must be a function');
+  }
+
+  let currentTheme = normalizeTheme(initialTheme || storage.getItem(storageKey));
+
+  function sync() {
+    applyTheme(currentTheme);
+    updateToggle({
+      theme: currentTheme,
+      label: currentTheme === DARK_THEME ? 'Switch to light mode' : 'Switch to dark mode',
+      pressed: currentTheme === DARK_THEME,
+    });
+  }
+
+  function setTheme(theme) {
+    currentTheme = normalizeTheme(theme);
+    storage.setItem(storageKey, currentTheme);
+    sync();
+    return currentTheme;
+  }
+
+  function toggleTheme() {
+    return setTheme(currentTheme === DARK_THEME ? LIGHT_THEME : DARK_THEME);
+  }
+
+  sync();
+
+  return {
+    getTheme: () => currentTheme,
+    setTheme,
+    toggleTheme,
+  };
+}
+
+module.exports = {
+  DARK_THEME,
+  LIGHT_THEME,
+  THEME_KEY,
+  createThemeToggleController,
+  normalizeTheme,
+};

test/theme-preference.test.js

@@ -0,0 +1,96 @@
+const {
+  DARK_THEME,
+  LIGHT_THEME,
+  createThemeToggleController,
+  normalizeTheme,
+} = require('../src/theme-preference');
+
+let passed = 0;
+let failed = 0;
+
+function assert(name, condition) {
+  if (condition) {
+    console.log(`  OK ${name}`);
+    passed++;
+  } else {
+    console.log(`  FAIL ${name}`);
+    failed++;
+  }
+}
+
+function createStorage(initial = {}) {
+  const values = { ...initial };
+  return {
+    getItem: (key) => values[key] || null,
+    setItem: (key, value) => {
+      values[key] = value;
+    },
+    values,
+  };
+}
+
+function testNormalizesUnknownThemes() {
+  assert('dark is preserved', normalizeTheme('dark') === DARK_THEME);
+  assert('unknown theme falls back to light', normalizeTheme('sepia') === LIGHT_THEME);
+}
+
+function testRestoresStoredPreference() {
+  const storage = createStorage({ theme: 'dark' });
+  const applied = [];
+  const toggleStates = [];
+
+  const controller = createThemeToggleController({
+    storage,
+    applyTheme: (theme) => applied.push(theme),
+    updateToggle: (state) => toggleStates.push(state),
+  });
+
+  assert('restores stored dark theme', controller.getTheme() === DARK_THEME);
+  assert('applies restored theme on start', applied[0] === DARK_THEME);
+  assert('marks toggle pressed in dark mode', toggleStates[0].pressed === true);
+}
+
+function testTogglesAndPersistsTheme() {
+  const storage = createStorage();
+  const applied = [];
+
+  const controller = createThemeToggleController({
+    storage,
+    applyTheme: (theme) => applied.push(theme),
+    updateToggle: () => {},
+  });
+
+  const nextTheme = controller.toggleTheme();
+
+  assert('toggles light to dark', nextTheme === DARK_THEME);
+  assert('persists dark preference', storage.values.theme === DARK_THEME);
+  assert('applies toggled theme', applied[applied.length - 1] === DARK_THEME);
+}
+
+function testCanSetThemeDirectly() {
+  const storage = createStorage();
+  const toggleStates = [];
+
+  const controller = createThemeToggleController({
+    storage,
+    applyTheme: () => {},
+    updateToggle: (state) => toggleStates.push(state),
+  });
+
+  controller.setTheme('dark');
+  controller.setTheme('light');
+
+  assert('sets light theme directly', controller.getTheme() === LIGHT_THEME);
+  assert('updates toggle label for light mode', toggleStates[toggleStates.length - 1].label === 'Switch to dark mode');
+  assert('marks toggle unpressed in light mode', toggleStates[toggleStates.length - 1].pressed === false);
+}
+
+console.log('\nTheme preference tests\n');
+
+testNormalizesUnknownThemes();
+testRestoresStoredPreference();
+testTogglesAndPersistsTheme();
+testCanSetThemeDirectly();
+
+console.log(`\nResults: ${passed} passed, ${failed} failed\n`);
+process.exit(failed > 0 ? 1 : 0);