fix(frontend): use unifiedSize on the new onboarding step's buttons

`size` and `color` are deprecated on Button; the new step copied them from the
survey steps above it. AGENTS.md: deprecated props survive at old call sites,
copying one forward is still a bug.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012fRjnaHLwjpHN84gNNxah9

frontend/src/routes/(root)/(logged)/user/(user)/onboarding/+page.svelte

@@ -383,19 +383,17 @@
 						<span></span>
 					{:else}
 						<Button
-							color="light"
-							variant="border"
+							variant="default"
+							unifiedSize="xs"
 							startIcon={{ icon: ArrowLeft }}
-							size="xs"
 							on:click={goToPreviousStep}
 						>
 							Previous
 						</Button>
 					{/if}
 					<Button
-						color="blue"
-						variant="contained"
-						size="lg"
+						variant="accent"
+						unifiedSize="md"
 						disabled={!!workspaceNameProblem}
 						on:click={confirmWorkspaceName}
 					>