feat(frontend): viewer zoom loads original and Space resets to fit

- in zoom, preload the full-resolution original and swap it in once loaded
  (preview is shown until then); revert to preview on zoom exit
- Space returns to the fitted view from any zoom level; from fit it goes to 100%
- derive zoom scale from the photo's true dimensions so 100% is accurate

frontend/src/lib/components/Viewer.svelte

@@ -42,6 +42,7 @@
 	let isFullscreen = $state(false);
 	let zoomed = $state(false);
 	let scale = $state(1);
+	let hiResLoaded = $state(false);
 
 	let showTop = $state(false);
 	let showBottom = $state(false);
@@ -60,17 +61,41 @@
 	let startL = 0;
 	let startT = 0;
 
+	const previewSrc = $derived(
+		fresh?.preview_url ?? fresh?.original_url ?? current?.preview_url ?? current?.original_url ?? ''
+	);
+	const fullSrc = $derived(fresh?.original_url ?? current?.original_url ?? '');
+	const displaySrc = $derived(zoomed && hiResLoaded && fullSrc ? fullSrc : previewSrc);
+
 	$effect(() => {
 		const id = current?.id;
 		fresh = null;
 		zoomed = false;
 		scale = 1;
+		hiResLoaded = false;
 		if (!id) return;
 		api<{ photo: Photo }>(`/photos/${id}`)
 			.then((data) => (fresh = data.photo))
 			.catch(() => {});
 	});
 
+	// In zoom mode, preload the full-resolution original and swap it in once loaded.
+	$effect(() => {
+		if (!zoomed || hiResLoaded) return;
+		const url = fullSrc;
+		if (!url) return;
+		const im = new Image();
+		let alive = true;
+		im.onload = () => {
+			if (alive) hiResLoaded = true;
+		};
+		im.src = url;
+		return () => {
+			alive = false;
+			im.onload = null;
+		};
+	});
+
 	$effect(() => {
 		const el = scroller;
 		if (!el) return;
@@ -205,10 +230,10 @@
 	}
 
 	function baseWidth(): number {
-		return imgEl?.naturalWidth || fresh?.width || current?.width || 0;
+		return fresh?.width || current?.width || imgEl?.naturalWidth || 0;
 	}
 	function baseHeight(): number {
-		return imgEl?.naturalHeight || fresh?.height || current?.height || 0;
+		return fresh?.height || current?.height || imgEl?.naturalHeight || 0;
 	}
 
 	function prev() {
@@ -219,8 +244,7 @@
 	}
 
 	function toggleActual() {
-		const atHundred = zoomed && Math.abs(scale - 1) < 0.01;
+		if (zoomed) {
-		if (atHundred) {
 			zoomed = false;
 			scale = 1;
 			if (scroller) {
@@ -434,10 +458,7 @@
 			{#if current}
 				<img
 					bind:this={imgEl}
-					src={fresh?.preview_url ??
+					src={displaySrc}
-						fresh?.original_url ??
-						current.preview_url ??
-						current.original_url}
 					alt={current.original_filename}
 					draggable="false"
 					style={zoomed ? `width: ${baseWidth() * scale}px; height: auto;` : ''}