Sleek. It is clear that you have paid a lot of attention from the beginning to the UX, which is great!

Just a quick feedback after a few minutes of use:

- Don't be worried by empty spaces, use them to strengthen the UI readability. For example, between text and images, around the "post a reply" area, between posts on mobile. Of course, spacing is relative, so you probably also have to reduce it a little in some areas (line-height, after the post top row, before the bottom row), to balance it and give components/areas the sense of a unique element.
And remember that spaces are perceived differently in light and dark modes.

- On mobile, pay attention to the use of the back button; the user expects to go back to the preview view, e.g., when you zoom an image or open the replies.

- The hamburger menu to the left is okay if you let the user open it with a horizontal swipe; otherwise, it is better to put it on the right.

Good work!