fix: center h1/h2/h3 headings in problem and solution content

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andrewboldi

src/lib/components/ProblemDisplay.svelte

@@ -32,4 +32,9 @@
 	.text :global(img:not(.latex)) {
 		filter: var(--img-filter, invert(0));
 	}
+	.text :global(h1),
+	.text :global(h2),
+	.text :global(h3) {
+		text-align: center;
+	}
 </style>

src/lib/components/SolutionDisplay.svelte

@@ -24,4 +24,9 @@
 	.text :global(img:not(.latex)) {
 		filter: var(--img-filter, invert(0));
 	}
+	.text :global(h1),
+	.text :global(h2),
+	.text :global(h3) {
+		text-align: center;
+	}
 </style>