Fix post navigation button functionality

src/site/html/static/js/post_overlay.js

@@ -148,11 +148,11 @@ function loadPostOverlay(shortcode, stateChangeType) {
 				overlay.element.querySelectorAll(".navigate-posts").forEach(button => {
 					button.addEventListener("click", async event => {
 						/** @type {HTMLButtonElement} */
-						//@ts-ignore
+						// @ts-ignore
 						const button = event.currentTarget
-						if (button.classList.contains("next")) {
+						if (button.hasAttribute("data-next")) {
 							navigate("next")
-						} else {
+						} else if (button.hasAttribute("data-previous")) {
 							navigate("previous")
 						}
 					})

src/site/pug/includes/post.pug

@@ -12,9 +12,9 @@ mixin post(post, headerWithNavigation)
 						else
 							= `@${post.getBasicOwner().username}`
 				if headerWithNavigation
-					button.navigate-posts
+					button(data-previous).navigate-posts
 						img(src="/static/img/arrow-circled.svg" alt="Previous post.").icon.previous
-					button.navigate-posts
+					button(data-next).navigate-posts
 						img(src="/static/img/arrow-circled.svg" alt="Next post.").icon
 			div.relative-box
 				div.scrolling-box