feat(#5): add icon presets, dark mode theme store, and icon-aware graph rendering

src/lib/stores/theme.ts

@@ -0,0 +1,55 @@
+import { writable } from 'svelte/store';
+
+export type ThemeMode = 'light' | 'dark';
+
+const STORAGE_KEY = 'ond-theme';
+
+function getInitialTheme(): ThemeMode {
+	if (typeof window === 'undefined') {
+		return 'light';
+	}
+	const stored = window.localStorage.getItem(STORAGE_KEY);
+	if (stored === 'light' || stored === 'dark') {
+		return stored;
+	}
+	return 'light';
+}
+
+function applyTheme(theme: ThemeMode) {
+	if (typeof document === 'undefined') {
+		return;
+	}
+	document.documentElement.setAttribute('data-theme', theme);
+}
+
+function createThemeStore() {
+	const store = writable<ThemeMode>('light');
+
+	return {
+		subscribe: store.subscribe,
+		initialize() {
+			const theme = getInitialTheme();
+			store.set(theme);
+			applyTheme(theme);
+		},
+		toggle() {
+			store.update((current) => {
+				const next = current === 'dark' ? 'light' : 'dark';
+				if (typeof window !== 'undefined') {
+					window.localStorage.setItem(STORAGE_KEY, next);
+				}
+				applyTheme(next);
+				return next;
+			});
+		},
+		set(theme: ThemeMode) {
+			if (typeof window !== 'undefined') {
+				window.localStorage.setItem(STORAGE_KEY, theme);
+			}
+			applyTheme(theme);
+			store.set(theme);
+		}
+	};
+}
+
+export const themeMode = createThemeStore();