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].

ios/LibNovel/LibNovel/Networking/APIClient.swift

@@ -348,7 +348,7 @@ actor APIClient {
     /// Delete a comment (and its replies) by ID. Only the owner can delete.
     func deleteComment(commentId: String) async throws {
         struct Empty: Decodable {}
-        let _: Empty = try await fetch("/api/comments/\(commentId)", method: "DELETE")
+        let _: Empty = try await fetch("/api/comment/\(commentId)", method: "DELETE")
     }
 }