fix: center artist handle on mobile (#650)

the handle-row uses flexbox which ignores text-align from the parent.
added justify-content: center in the mobile media query.

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Co-authored-by: Claude Opus 4.5 <[e-mail]>

frontend/src/routes/u/[handle]/+page.svelte

···
 			text-align: center;
 		}
 
+		.handle-row {
+			justify-content: center;
+		}
+
 		.artist-actions-desktop {
 			display: none;
 		}