feat: add image lightbox for full-size preview on click

Clicking a thumbnail in the editor, closed-panel view, or note card
opens a full-screen overlay with the full-size image. Closes on Escape,
background click, or X button. Escape is scoped so it won't close the
editor underneath.

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

src/lib/components/ImageLightbox.svelte

@@ -0,0 +1,49 @@
+<script lang="ts">
+	import XIcon from 'lucide-svelte/icons/x';
+
+	interface Props {
+		src: string;
+		alt: string;
+		onClose: () => void;
+	}
+
+	const { src, alt, onClose }: Props = $props();
+
+	function handleKeydown(e: KeyboardEvent) {
+		if (e.key === 'Escape') {
+			e.stopPropagation();
+			onClose();
+		}
+	}
+
+	function handleBackgroundClick(e: MouseEvent) {
+		if (e.target === e.currentTarget) {
+			onClose();
+		}
+	}
+</script>
+
+<svelte:window onkeydown={handleKeydown} />
+
+<!-- svelte-ignore a11y_no_static_element_interactions -->
+<div
+	class="fixed inset-0 z-50 flex items-center justify-center bg-black/80 animate-[fade-in_150ms_ease-out]"
+	onclick={handleBackgroundClick}
+	data-testid="image-lightbox"
+>
+	<button
+		onclick={onClose}
+		class="absolute right-4 top-4 rounded-sm p-2 text-white hover:bg-white/10"
+		aria-label="Close lightbox"
+		data-testid="lightbox-close"
+	>
+		<XIcon class="h-6 w-6" />
+	</button>
+
+	<img
+		{src}
+		{alt}
+		class="max-h-[90vh] max-w-[90vw] object-contain rounded-sm animate-[pop-in_150ms_ease-out]"
+		data-testid="lightbox-image"
+	/>
+</div>