Add multi-select filters, light/dark theme, and skill ZIP downloads

- Add multi-select dropdown component for all filter fields
- Implement light/dark theme toggle with system preference detection
- Add client-side ZIP download for skills using JSZip
- Include file lists in skills metadata for download feature
- Add title tooltips to multi-select options for long values
- Update all pages with consistent theme toggle in header

website/js/theme.js

@@ -0,0 +1,74 @@
+/**
+ * Theme management for the Awesome Copilot website
+ * Supports light/dark mode with user preference storage
+ */
+
+const THEME_KEY = 'awesome-copilot-theme';
+
+/**
+ * Get the current theme preference
+ * Priority: localStorage > system preference > dark (default)
+ */
+function getThemePreference() {
+  const stored = localStorage.getItem(THEME_KEY);
+  if (stored === 'light' || stored === 'dark') {
+    return stored;
+  }
+  // Check system preference
+  if (window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches) {
+    return 'light';
+  }
+  return 'dark';
+}
+
+/**
+ * Apply theme to the document
+ */
+function applyTheme(theme) {
+  if (theme === 'light') {
+    document.documentElement.setAttribute('data-theme', 'light');
+  } else {
+    document.documentElement.setAttribute('data-theme', 'dark');
+  }
+}
+
+/**
+ * Toggle between light and dark theme
+ */
+function toggleTheme() {
+  const current = document.documentElement.getAttribute('data-theme');
+  const newTheme = current === 'light' ? 'dark' : 'light';
+  applyTheme(newTheme);
+  localStorage.setItem(THEME_KEY, newTheme);
+}
+
+/**
+ * Initialize theme on page load
+ */
+function initTheme() {
+  // Apply theme immediately to prevent flash
+  const theme = getThemePreference();
+  applyTheme(theme);
+
+  // Listen for system theme changes
+  if (window.matchMedia) {
+    window.matchMedia('(prefers-color-scheme: light)').addEventListener('change', (e) => {
+      // Only auto-switch if user hasn't set a preference
+      const stored = localStorage.getItem(THEME_KEY);
+      if (!stored) {
+        applyTheme(e.matches ? 'light' : 'dark');
+      }
+    });
+  }
+}
+
+// Initialize theme immediately (before DOM ready to prevent flash)
+initTheme();
+
+// Setup toggle button after DOM ready
+document.addEventListener('DOMContentLoaded', () => {
+  const toggleBtn = document.getElementById('theme-toggle');
+  if (toggleBtn) {
+    toggleBtn.addEventListener('click', toggleTheme);
+  }
+});