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/components/ConfigSidebar.svelte

@@ -4,6 +4,8 @@
 	import { SessionConfigSchema } from '$lib/proto/llm_multiverse/v1/orchestrator_pb';
 	import { create } from '@bufbuild/protobuf';
 	import { presetStore } from '$lib/stores/presets.svelte';
+	import { isNonDefaultConfig } from '$lib/utils/sessionConfig';
+	import Backdrop from '$lib/components/Backdrop.svelte';
 
 	let {
 		config,
@@ -37,11 +39,7 @@
 		{ value: OverrideLevel.ALL, label: 'All', description: 'No enforcement' }
 	];
 
-	const isNonDefault = $derived(
-		config.overrideLevel !== OverrideLevel.NONE ||
-			config.disabledTools.length > 0 ||
-			config.grantedPermissions.length > 0
-	);
+	const isNonDefault = $derived(isNonDefaultConfig(config));
 
 	function setOverrideLevel(level: OverrideLevel) {
 		const updated = create(SessionConfigSchema, {
@@ -120,16 +118,8 @@
 	}
 </script>
 
-<!-- Mobile overlay backdrop -->
 {#if onClose}
-	<div
-		class="fixed inset-0 z-40 bg-black/50 md:hidden"
-		role="button"
-		tabindex="-1"
-		onclick={onClose}
-		onkeydown={(e) => { if (e.key === 'Escape') onClose?.(); }}
-		aria-label="Close config sidebar"
-	></div>
+	<Backdrop onClose={onClose} />
 {/if}
 
 <aside