feat(ui): add light theme toggle

src/lib/ui/theme.ts

@@ -0,0 +1,54 @@
+export const UI_THEME_STORAGE_KEY = "dashboard-ui-theme";
+
+export type UiTheme = "dark" | "light";
+
+type ReadableThemeStorage =
+  | { getItem(key: string): string | null }
+  | { get(key: string): string | undefined };
+
+type WritableThemeStorage =
+  | { setItem(key: string, value: string): void }
+  | { set(key: string, value: string): unknown };
+
+export function isUiTheme(value: unknown): value is UiTheme {
+  return value === "dark" || value === "light";
+}
+
+export function getNextUiTheme(theme: UiTheme): UiTheme {
+  return theme === "dark" ? "light" : "dark";
+}
+
+export function resolveInitialUiTheme(
+  storage?: ReadableThemeStorage | null,
+  fallback: UiTheme = "dark",
+): UiTheme {
+  const stored = readStoredTheme(storage);
+
+  return isUiTheme(stored) ? stored : fallback;
+}
+
+export function persistUiTheme(
+  theme: UiTheme,
+  storage?: WritableThemeStorage | null,
+): void {
+  if (!storage) return;
+
+  if ("setItem" in storage) {
+    storage.setItem(UI_THEME_STORAGE_KEY, theme);
+    return;
+  }
+
+  storage.set(UI_THEME_STORAGE_KEY, theme);
+}
+
+function readStoredTheme(
+  storage?: ReadableThemeStorage | null,
+): string | undefined {
+  if (!storage) return undefined;
+
+  if ("getItem" in storage) {
+    return storage.getItem(UI_THEME_STORAGE_KEY) ?? undefined;
+  }
+
+  return storage.get(UI_THEME_STORAGE_KEY);
+}