Consolidate unvote link clearing logic into VotingViewModel

Move the logic to clear the unvote link after a successful unvote operation from the UI layer into VotingViewModel.unvote(). This eliminates duplicate code that was present in both the swipe action and handleUnvote() method.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: weiran

Features/Comments/Sources/Comments/CommentsComponents.swift

@@ -63,12 +63,7 @@ struct CommentsContentView: View {
                                         var mutablePost = post
                                         await votingViewModel.unvote(post: &mutablePost)
                                         await MainActor.run {
-                                            if !mutablePost.upvoted {
-                                                if let existingLinks = mutablePost.voteLinks {
-                                                    mutablePost.voteLinks = VoteLinks(upvote: existingLinks.upvote, unvote: nil)
-                                                }
-                                                viewModel.post = mutablePost
-                                            }
+                                            viewModel.post = mutablePost
                                         }
                                     }
                                 } label: {
@@ -287,9 +282,6 @@ struct PostHeader: View {
         let wasUnvoted = !mutablePost.upvoted
 
         if wasUnvoted {
-            if let existingLinks = mutablePost.voteLinks {
-                mutablePost.voteLinks = VoteLinks(upvote: existingLinks.upvote, unvote: nil)
-            }
             await MainActor.run {
                 onPostUpdated(mutablePost)
             }

Shared/Sources/Shared/ViewModels/VotingViewModel.swift

@@ -86,6 +86,11 @@ public final class VotingViewModel {
         post.upvoted = false
         post.score -= 1
 
+        // Clear unvote link after successful unvote
+        if let existingLinks = post.voteLinks {
+            post.voteLinks = VoteLinks(upvote: existingLinks.upvote, unvote: nil)
+        }
+
         isVoting = true
         lastError = nil