Add theme store for dark/light mode management

frontend/src/lib/theme.ts

@@ -0,0 +1,60 @@
+import { writable } from 'svelte/store';
+
+export type Theme = 'light' | 'dark';
+
+// Get initial theme from localStorage or system preference
+function getInitialTheme(): Theme {
+  if (typeof window === 'undefined') return 'light';
+  
+  const stored = localStorage.getItem('theme') as Theme;
+  if (stored === 'light' || stored === 'dark') {
+    return stored;
+  }
+
+  // Check system preference
+  if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
+    return 'dark';
+  }
+
+  return 'light';
+}
+
+// Create the theme store
+function createThemeStore() {
+  const { subscribe, set, update } = writable<Theme>(getInitialTheme());
+
+  return {
+    subscribe,
+    set: (theme: Theme) => {
+      if (typeof window !== 'undefined') {
+        localStorage.setItem('theme', theme);
+        document.documentElement.setAttribute('data-theme', theme);
+      }
+      set(theme);
+    },
+    toggle: () => {
+      update(current => {
+        const newTheme = current === 'light' ? 'dark' : 'light';
+        if (typeof window !== 'undefined') {
+          localStorage.setItem('theme', newTheme);
+          document.documentElement.setAttribute('data-theme', newTheme);
+        }
+        return newTheme;
+      });
+    },
+    init: () => {
+      const theme = getInitialTheme();
+      if (typeof window !== 'undefined') {
+        document.documentElement.setAttribute('data-theme', theme);
+      }
+      set(theme);
+    }
+  };
+}
+
+export const theme = createThemeStore();
+
+// Initialize theme on module load
+if (typeof window !== 'undefined') {
+  theme.init();
+}