fix(frontend): correct diff direction in history panel

versions array is ascending (oldest first), so the previous version
is at idx-1, not idx+1. Using idx+1 caused added/removed to be swapped,
showing new text as red and old text as green.

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -217,7 +217,7 @@ async function selectVersion(versionId: string) {
 	try {
 		const idx = versions.findIndex((v) => v.id === versionId);
 		const newerSnap = await fetchSnapshot(versionId);
-		const olderSnap = idx + 1 < versions.length ? await fetchSnapshot(versions[idx + 1].id) : null;
+		const olderSnap = idx > 0 ? await fetchSnapshot(versions[idx - 1].id) : null;
 		const entries = buildDiff(olderSnap, newerSnap);
 		if (entries.length === 0) {
 			noDiff = true;