feat: add dark/light theme toggle with system preference support (#18)

Add theme switching with three modes (system/light/dark) using Tailwind v4
class-based dark mode. Theme preference is persisted in localStorage and
defaults to the system's prefers-color-scheme. All components updated with
dark: variants for consistent dark mode rendering including SVG elements.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/stores/theme.svelte.ts

@@ -0,0 +1,86 @@
+export type ThemeMode = 'light' | 'dark' | 'system';
+
+const STORAGE_KEY = 'llm-multiverse-theme';
+
+function createThemeStore() {
+	let mode: ThemeMode = $state('system');
+	let resolvedDark = $state(false);
+
+	function applyTheme(isDark: boolean) {
+		if (typeof document === 'undefined') return;
+		const root = document.documentElement;
+		// Add transition class for smooth switching
+		root.classList.add('theme-transition');
+		if (isDark) {
+			root.classList.add('dark');
+		} else {
+			root.classList.remove('dark');
+		}
+		// Remove transition class after animation completes
+		setTimeout(() => {
+			root.classList.remove('theme-transition');
+		}, 300);
+	}
+
+	function getSystemPreference(): boolean {
+		if (typeof window === 'undefined') return false;
+		return window.matchMedia('(prefers-color-scheme: dark)').matches;
+	}
+
+	function init() {
+		if (typeof window === 'undefined') return;
+
+		// Load saved preference
+		const saved = localStorage.getItem(STORAGE_KEY) as ThemeMode | null;
+		if (saved === 'light' || saved === 'dark' || saved === 'system') {
+			mode = saved;
+		} else {
+			mode = 'system';
+		}
+
+		// Listen for system preference changes
+		const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+		mediaQuery.addEventListener('change', (e) => {
+			if (mode === 'system') {
+				resolvedDark = e.matches;
+				applyTheme(resolvedDark);
+			}
+		});
+
+		// Apply initial theme
+		resolvedDark =
+			mode === 'dark' ? true : mode === 'light' ? false : getSystemPreference();
+		applyTheme(resolvedDark);
+	}
+
+	function setMode(newMode: ThemeMode) {
+		mode = newMode;
+		if (typeof window !== 'undefined') {
+			localStorage.setItem(STORAGE_KEY, newMode);
+		}
+		resolvedDark =
+			newMode === 'dark' ? true : newMode === 'light' ? false : getSystemPreference();
+		applyTheme(resolvedDark);
+	}
+
+	function cycle() {
+		const order: ThemeMode[] = ['system', 'light', 'dark'];
+		const idx = order.indexOf(mode);
+		const next = order[(idx + 1) % order.length];
+		setMode(next);
+	}
+
+	return {
+		get mode() {
+			return mode;
+		},
+		get isDark() {
+			return resolvedDark;
+		},
+		init,
+		setMode,
+		cycle
+	};
+}
+
+export const themeStore = createThemeStore();