fix: add missing DELETE handler and fix comment delete/vote URLs (web + iOS)

The /api/comments/[id] delete route was never created; the deleteComment helper
in pocketbase.ts existed but was unreachable. Added DELETE /api/comment/[id]
route handler alongside the existing vote route. Updated CommentsSection.svelte
and iOS APIClient to use /api/comment/{id} for both delete and (already fixed)
vote, keeping all comment-mutation endpoints under the singular /api/comment/
prefix to avoid SvelteKit route conflicts with /api/comments/[slug].

ui/src/lib/components/CommentsSection.svelte

@@ -156,7 +156,7 @@
 		if (deletingIds.has(commentId)) return;
 		deletingIds = new Set([...deletingIds, commentId]);
 		try {
-			const res = await fetch(`/api/comments/${commentId}`, { method: 'DELETE' });
+			const res = await fetch(`/api/comment/${commentId}`, { method: 'DELETE' });
 			if (!res.ok) return;
 			if (parentId) {
 				// Remove reply from parent