Add Back to home at the end of each shelf (#75)

After Other shelves, offer a clear exit to the pie-chart home so
readers who finish a shelf do not have to scroll back to the header.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: jsolly

src/components/app/ShelfView.svelte

@@ -221,5 +221,16 @@
 				{/each}
 			</ul>
 		</nav>
+
+		<div class="mt-10 border-t pt-6 sm:mt-12">
+			<Button
+				size="lg"
+				variant="outline"
+				class="w-full sm:h-11 sm:text-base"
+				href="/"
+			>
+				Back to home
+			</Button>
+		</div>
 	</div>
 {/if}