fix(frontend): prevent avatar layout shift while loading

The .avatar img in User.vue relied solely on the width/height HTML
attributes for sizing. Those are presentational hints with zero CSS
specificity, so Bulma's global reset (img { height: auto; max-width: 100% })
overrode them. While avatarSrc was still resolving (initial src=""),
the browser had no intrinsic dimensions to compute the auto height from
and fell back to the broken-image box (~96px in Chrome), then snapped
to the real size once the blob URL loaded.

Set inline-size/block-size explicitly via a CSS custom property bound
to the avatarSize prop so the rendered size is locked regardless of
load state or the Bulma reset.

frontend/src/components/misc/User.vue

@@ -2,6 +2,7 @@
 	<div
 		class="user"
 		:class="{'is-inline': isInline}"
+		:style="{'--avatar-size': `${avatarSize}px`}"
 	>
 		<span class="avatar-wrapper">
 			<img
@@ -74,6 +75,8 @@ watch(() => [props.user, props.avatarSize], loadAvatar, { immediate: true })
 }
 
 .avatar {
+	inline-size: var(--avatar-size);
+	block-size: var(--avatar-size);
 	border-radius: 100%;
 	vertical-align: middle;
 }