refactor: code review improvements — fix bugs, extract shared utilities, add README

- Fix reactivity bug: use SvelteMap instead of Map in sessions store
- Fix theme listener memory leak: guard against double-init, return cleanup function
- Fix transport singleton ignoring different endpoints
- Fix form/button type mismatch in MessageInput
- Add safer retry validation in chat page
- Extract shared utilities: date formatting, session config check, result source config
- Extract shared components: Backdrop, PageHeader
- Extract orchestration composable from chat page (310→85 lines of script)
- Consolidate AuditTimeline switch functions into config record
- Move sample data to dedicated module
- Add dark mode support for LineageTree SVG colors
- Memoize leaf count computation in LineageTree
- Update README with usage guide and project structure

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

src/lib/stores/theme.svelte.ts

@@ -5,6 +5,9 @@ const STORAGE_KEY = 'llm-multiverse-theme';
 function createThemeStore() {
 	let mode: ThemeMode = $state('system');
 	let resolvedDark = $state(false);
+	let initialized = false;
+	let mediaQuery: MediaQueryList | null = null;
+	let mediaListener: ((e: MediaQueryListEvent) => void) | null = null;
 
 	function applyTheme(isDark: boolean) {
 		if (typeof document === 'undefined') return;
@@ -27,8 +30,10 @@ function createThemeStore() {
 		return window.matchMedia('(prefers-color-scheme: dark)').matches;
 	}
 
-	function init() {
+	function init(): (() => void) | undefined {
 		if (typeof window === 'undefined') return;
+		if (initialized) return;
+		initialized = true;
 
 		// Load saved preference
 		const saved = localStorage.getItem(STORAGE_KEY) as ThemeMode | null;
@@ -39,18 +44,26 @@ function createThemeStore() {
 		}
 
 		// Listen for system preference changes
-		const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
-		mediaQuery.addEventListener('change', (e) => {
+		mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+		mediaListener = (e: MediaQueryListEvent) => {
 			if (mode === 'system') {
 				resolvedDark = e.matches;
 				applyTheme(resolvedDark);
 			}
-		});
+		};
+		mediaQuery.addEventListener('change', mediaListener);
 
 		// Apply initial theme
 		resolvedDark =
 			mode === 'dark' ? true : mode === 'light' ? false : getSystemPreference();
 		applyTheme(resolvedDark);
+
+		return () => {
+			if (mediaQuery && mediaListener) {
+				mediaQuery.removeEventListener('change', mediaListener);
+			}
+			initialized = false;
+		};
 	}
 
 	function setMode(newMode: ThemeMode) {