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/utils/date.ts

@@ -0,0 +1,19 @@
+/**
+ * Formats a date as a relative string: "Today", "Yesterday", "N days ago", or locale date.
+ */
+export function formatRelativeDate(date: Date): string {
+	const now = new Date();
+	const diff = now.getTime() - date.getTime();
+	const days = Math.floor(diff / (1000 * 60 * 60 * 24));
+	if (days === 0) return 'Today';
+	if (days === 1) return 'Yesterday';
+	if (days < 7) return `${days} days ago`;
+	return date.toLocaleDateString();
+}
+
+/**
+ * Formats a date as a short string like "Mar 12, 2026".
+ */
+export function formatShortDate(date: Date): string {
+	return date.toLocaleDateString([], { month: 'short', day: 'numeric', year: 'numeric' });
+}