p13n: minor SeaPen usability improvements

VC Background does not need to fetch a high resolution image,
so do not show the loading state. When error occurs, fade in
the error illustration. Apply a minimum loading time of 1s
so the placeholder loading graphic does not flash for <100ms
when disconnected from internet or other immediate error.

Add a 2px transparent border on the loading sparkle to prevent
bleedthrough when hovering over the image thumbnails.

Bug: b:333773429
Change-Id: I8b490e5e6dbe726c36e4303827c77a98e9063400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5447234
Reviewed-by: Jason Thai <jasontt@chromium.org>
Commit-Queue: Jeffrey Young <cowmoo@google.com>
Cr-Commit-Position: refs/heads/main@{#1286824}