feat: add interactive code panel with step explanations (Phase 4)

Implement code visualization with syntax highlighting and interactive step exploration.

New Components:
- CodePanel: Shiki-highlighted flow code with clickable lines
- ExplanationPanel: Shows step dependencies, inputs, and returns on click
- flow-code.ts: Simplified educational code snippet with line-to-step mapping

Enhanced Components:
- DAGVisualization: Added click handling and selection states
- DebugPanel: Auto-scroll to selected step with smooth animations
- Page layout: 55/45 split, all panels synchronized via step-selected events

Interactions:
- Click code line or DAG node → shows explanation + scrolls debug panel
- Visual states: active (executing) vs selected (clicked)
- ESC or outside click dismisses explanation panel

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: jumski

apps/demo/src/lib/components/CodePanel.svelte

@@ -0,0 +1,280 @@
+<script lang="ts">
+	import { onMount, createEventDispatcher } from 'svelte';
+	import { codeToHtml } from 'shiki';
+	import { FLOW_CODE, getStepFromLine, FLOW_SECTIONS } from '$lib/data/flow-code';
+	import type { createFlowState } from '$lib/stores/pgflow-state-improved.svelte';
+	import StatusBadge from '$lib/components/StatusBadge.svelte';
+
+	interface Props {
+		flowState: ReturnType<typeof createFlowState>;
+		selectedStep: string | null;
+		hoveredStep: string | null;
+	}
+
+	let { flowState, selectedStep, hoveredStep }: Props = $props();
+
+	const dispatch = createEventDispatcher<{
+		'step-selected': { stepSlug: string };
+		'step-hovered': { stepSlug: string | null };
+	}>();
+
+	let highlightedCode = $state('');
+	let codeContainer: HTMLElement | undefined = $state(undefined);
+
+	// Calculate step blocks (groups of lines) for status icon positioning
+	const stepBlocks = $derived.by(() => {
+		const blocks: Array<{ stepSlug: string; startLine: number; endLine: number }> = [];
+
+		for (const [stepSlug, section] of Object.entries(FLOW_SECTIONS)) {
+			if (stepSlug === 'flow_config') continue; // Skip flow config
+			if (section.startLine !== undefined && section.endLine !== undefined) {
+				blocks.push({ stepSlug, startLine: section.startLine, endLine: section.endLine });
+			}
+		}
+
+		return blocks;
+	});
+
+	// Helper to get status for a step badge
+	function getStepStatus(stepSlug: string): string | null {
+		const status = flowState.stepStatuses[stepSlug];
+		const hasFlowStarted = flowState.status !== 'idle';
+
+		// Don't show badge if flow hasn't started yet
+		if (!hasFlowStarted) {
+			return null;
+		}
+
+		// If flow has started but this step has no status yet, show as created
+		if (!status) {
+			return 'created';
+		}
+
+		return status;
+	}
+
+	onMount(async () => {
+		// Generate syntax highlighted HTML using Shiki
+		highlightedCode = await codeToHtml(FLOW_CODE, {
+			lang: 'typescript',
+			theme: 'night-owl',
+			transformers: [
+				{
+					line(node, line) {
+						// Add .line class to each line for click handling
+						node.properties.class = 'line';
+					}
+				}
+			]
+		});
+
+		// Add click handlers to lines after rendering - need small delay
+		setTimeout(() => {
+			if (codeContainer) {
+				setupClickHandlers();
+			}
+		}, 50);
+	});
+
+	function setupClickHandlers() {
+		if (!codeContainer) return;
+
+		// Find all line elements
+		const lines = codeContainer.querySelectorAll('.line');
+		lines.forEach((line, index) => {
+			const lineNumber = index + 1;
+			const stepSlug = getStepFromLine(lineNumber);
+
+			// Set data-step attribute for all lines (including flow_config)
+			if (stepSlug) {
+				(line as HTMLElement).setAttribute('data-step', stepSlug);
+				(line as HTMLElement).setAttribute('data-line', String(lineNumber));
+				(line as HTMLElement).style.cursor = 'pointer';
+
+				// Click handler
+				line.addEventListener('click', () => {
+					console.log('CodePanel: Line clicked, stepSlug:', stepSlug);
+					// Clear hover state before navigating
+					dispatch('step-hovered', { stepSlug: null });
+
+					// All sections (including flow_config) dispatch their slug
+					dispatch('step-selected', { stepSlug });
+				});
+
+				// Hover handlers - dispatch hover events
+				line.addEventListener('mouseenter', () => {
+					dispatch('step-hovered', { stepSlug });
+				});
+
+				line.addEventListener('mouseleave', () => {
+					dispatch('step-hovered', { stepSlug: null });
+				});
+			}
+		});
+	}
+
+	// Update line highlighting and borders based on step status, selected, and hovered steps
+	$effect(() => {
+		// Explicitly track these dependencies
+		const currentSelectedStep = selectedStep;
+		const currentHoveredStep = hoveredStep;
+		const currentStepStatuses = flowState.stepStatuses;
+
+		if (!codeContainer) return;
+
+		const lines = codeContainer.querySelectorAll('.line');
+		lines.forEach((line) => {
+			const stepSlug = (line as HTMLElement).getAttribute('data-step');
+			(line as HTMLElement).classList.remove('line-selected', 'line-hovered', 'line-dimmed');
+
+			if (stepSlug) {
+				// Dimming: dim all lines except selected when selecting (including flow_config)
+				if (currentSelectedStep && stepSlug !== currentSelectedStep) {
+					(line as HTMLElement).classList.add('line-dimmed');
+				}
+
+				// Hovered state (hovering) - blue highlight, no dimming (including flow_config)
+				if (currentHoveredStep && stepSlug === currentHoveredStep) {
+					(line as HTMLElement).classList.add('line-hovered');
+				}
+
+				// Selected state (clicked) - blue background with dimming (not for flow_config)
+				if (currentSelectedStep && stepSlug === currentSelectedStep) {
+					(line as HTMLElement).classList.add('line-selected');
+				}
+			}
+		});
+	});
+</script>
+
+<div class="code-panel-wrapper">
+	<div class="code-panel" bind:this={codeContainer}>
+		{@html highlightedCode}
+
+		<!-- Step status icons overlaid on code blocks -->
+		{#each stepBlocks as block}
+			{@const stepStatus = getStepStatus(block.stepSlug)}
+			{#if stepStatus}
+				{@const blockHeight = (block.endLine - block.startLine + 1) * 1.5}
+				{@const blockTop = (block.startLine - 1) * 1.5}
+				{@const iconTop = blockTop + blockHeight / 2}
+				<div
+					class="step-status-container"
+					data-step={block.stepSlug}
+					data-start-line={block.startLine}
+					style="top: calc({iconTop}em + 12px);"
+				>
+					<span class="status-label status-{stepStatus}">{stepStatus}</span>
+					<StatusBadge status={stepStatus} variant="icon-only" size="xl" />
+				</div>
+			{/if}
+		{/each}
+	</div>
+</div>
+
+<style>
+	.code-panel-wrapper {
+		position: relative;
+	}
+
+	.code-panel {
+		overflow-x: auto;
+		border-radius: 5px;
+		font-size: 15px;
+		background: #0d1117;
+		position: relative;
+	}
+
+	/* Override Shiki's default pre styling */
+	.code-panel :global(pre) {
+		margin: 0;
+		padding: 12px 0;
+		background: #0d1117 !important;
+		border-radius: 5px;
+		line-height: 1.5;
+	}
+
+	.code-panel :global(code) {
+		font-family: 'Fira Code', 'Monaco', 'Menlo', 'Courier New', monospace;
+	}
+
+	/* Line styling */
+	.code-panel :global(.line) {
+		display: inline-block;
+		width: 100%;
+		padding: 0 12px;
+		transition: background-color 0.2s ease;
+	}
+
+	/* Empty lines need content for background to show */
+	.code-panel :global(.line:empty::after) {
+		content: ' ';
+		display: inline-block;
+	}
+
+	/* Clickable lines */
+	.code-panel :global(.line[data-step]) {
+		cursor: pointer;
+	}
+
+	/* Dimmed (when another step is selected) - lowest priority */
+	.code-panel :global(.line-dimmed) {
+		opacity: 0.4;
+		transition: opacity 200ms ease;
+	}
+
+	/* Ensure non-dimmed lines also transition smoothly */
+	.code-panel :global(.line) {
+		transition: opacity 200ms ease, background-color 200ms ease;
+	}
+
+	/* Hover state - opaque blue highlight */
+	.code-panel :global(.line-hovered) {
+		background-color: rgba(88, 166, 255, 0.15) !important;
+		opacity: 1 !important;
+	}
+
+	/* Selected step (clicked by user) - stronger blue background */
+	.code-panel :global(.line-selected) {
+		background-color: rgba(88, 166, 255, 0.22) !important;
+		opacity: 1 !important;
+	}
+
+	/* Step status container */
+	.step-status-container {
+		position: absolute;
+		right: 16px;
+		transform: translateY(-50%);
+		z-index: 10;
+		pointer-events: none;
+		transition: all 0.3s ease;
+		display: flex;
+		align-items: center;
+		gap: 8px;
+	}
+
+	/* Status labels */
+	.status-label {
+		font-size: 0.875rem;
+		font-weight: 600;
+		text-transform: lowercase;
+		font-family: 'Fira Code', 'Monaco', 'Menlo', 'Courier New', monospace;
+		line-height: 1;
+	}
+
+	.status-completed {
+		color: #20a56f;
+	}
+
+	.status-started {
+		color: #5b8def;
+	}
+
+	.status-failed {
+		color: #f08060;
+	}
+
+	.status-created {
+		color: #607b75;
+	}
+</style>