bugfix: the whole theming structure overhauled. light/dark mode icon added.

frontend/src/lib/theme.ts

@@ -0,0 +1,30 @@
+export const THEME_STORAGE_KEY = 'dyolink-theme';
+
+export type ThemeMode = 'light' | 'dark';
+
+export function getStoredTheme(): ThemeMode {
+  if (typeof window === 'undefined') return 'dark';
+  try {
+    const v = localStorage.getItem(THEME_STORAGE_KEY);
+    if (v === 'light' || v === 'dark') return v;
+  } catch {
+    /* ignore */
+  }
+  return 'dark';
+}
+
+export function applyTheme(mode: ThemeMode) {
+  if (typeof document === 'undefined') return;
+  document.documentElement.setAttribute('data-theme', mode);
+  try {
+    localStorage.setItem(THEME_STORAGE_KEY, mode);
+  } catch {
+    /* ignore */
+  }
+}
+
+export function toggleTheme(): ThemeMode {
+  const next: ThemeMode = getStoredTheme() === 'dark' ? 'light' : 'dark';
+  applyTheme(next);
+  return next;
+}