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/sessions.svelte.ts

@@ -1,3 +1,4 @@
+import { SvelteMap } from 'svelte/reactivity';
 import type { ChatMessage } from '$lib/types';
 
 export interface Session {
@@ -10,24 +11,24 @@ export interface Session {
 const STORAGE_KEY = 'llm-multiverse-sessions';
 const ACTIVE_SESSION_KEY = 'llm-multiverse-active-session';
 
-function loadSessions(): Map<string, Session> {
-	if (typeof localStorage === 'undefined') return new Map();
+function loadSessions(): SvelteMap<string, Session> {
+	if (typeof localStorage === 'undefined') return new SvelteMap();
 	try {
 		const raw = localStorage.getItem(STORAGE_KEY);
-		if (!raw) return new Map();
+		if (!raw) return new SvelteMap();
 		const arr: [string, Session][] = JSON.parse(raw);
-		return new Map(
+		return new SvelteMap(
 			arr.map(([id, s]) => [
 				id,
 				{ ...s, createdAt: new Date(s.createdAt), messages: s.messages.map(m => ({ ...m, timestamp: new Date(m.timestamp) })) }
 			])
 		);
 	} catch {
-		return new Map();
+		return new SvelteMap();
 	}
 }
 
-function saveSessions(sessions: Map<string, Session>) {
+function saveSessions(sessions: SvelteMap<string, Session>) {
 	if (typeof localStorage === 'undefined') return;
 	localStorage.setItem(STORAGE_KEY, JSON.stringify([...sessions.entries()]));
 }
@@ -43,7 +44,7 @@ function saveActiveSessionId(id: string) {
 }
 
 function createSessionStore() {
-	const sessions = $state<Map<string, Session>>(loadSessions());
+	const sessions = $state<SvelteMap<string, Session>>(loadSessions());
 	let activeSessionId = $state<string | null>(loadActiveSessionId());
 
 	function createSession(id?: string): Session {