Disable placeholder animation

## Issue
https://www.notion.so/Performance-Fixes-927f825a5d674bd09323830be1d263af#1beab2fee011421492b56b88f68681a3

We currently lazy-load the tiles in the category sections (but not the sections themselves, because we want to retain scroll position on Back action). This puts gray placeholders until the section is visible on screen.  which turns out to be quite expensive, because the placeholders are animated, so we have a perpetual animation in the background after the homepage loads + user did not scroll.

## Change
Just disable the barely-noticeable animation for now.

There are alternatives, but probably not worth polluting the code with:
- Just like the thumbnails, use intersection observer to decide when to animate.
- Find solution to the "lazy load section + need to retain scroll position".

ui/scss/init/_mixins.scss

@@ -186,7 +186,7 @@
 }
 
 @mixin placeholder {
-  animation: pulse 2s infinite ease-in-out;
+  // animation: pulse 2s infinite ease-in-out;
   background-color: var(--color-placeholder-background);
   border-radius: var(--card-radius);
   border-width: 0;