feat: styled error screen (#8930)

* Styled error screen

* Make details margin auto

* Update boot.css

* Replace fontawesome with tabler svg

* Remove hr

* Add new style to flush screen

* Rename to `error.css`

packages/backend/src/server/web/views/flush.pug

@@ -1,6 +1,12 @@
 doctype html
 
 html
+	head
+		meta(charset='utf-8')
+		meta(name='application-name' content='Misskey')
+		title Flushing Misskey
+		style
+			include ../error.css
 	#msg
 	script.
 		const msg = document.getElementById('msg');