feat(web): polish sidebar shell and authentication UI

apps/web/src/services/theme-storage.ts

@@ -0,0 +1,29 @@
+const THEME_STORAGE_KEY = "todolist.web.theme";
+
+export type ThemeMode = "light" | "dark";
+
+function isThemeMode(value: string | null): value is ThemeMode {
+  return value === "light" || value === "dark";
+}
+
+export function loadThemeMode(): ThemeMode {
+  const savedTheme = window.localStorage.getItem(THEME_STORAGE_KEY);
+  if (isThemeMode(savedTheme)) {
+    return savedTheme;
+  }
+
+  if (window.matchMedia("(prefers-color-scheme: dark)").matches) {
+    return "dark";
+  }
+
+  return "light";
+}
+
+export function saveThemeMode(mode: ThemeMode): void {
+  window.localStorage.setItem(THEME_STORAGE_KEY, mode);
+}
+
+export function applyThemeMode(mode: ThemeMode): void {
+  document.documentElement.classList.toggle("dark", mode === "dark");
+  document.documentElement.style.colorScheme = mode;
+}