feat(nostr-feed): Add submission voting with persistence

- Implement downvote() for submissions using dislikeEvent
- Add refreshAllSubmissionVotes() to update all votes after EOSE
- Refresh submission votes after loading reactions in both subscribe methods
- Fixes vote state not displaying correctly on page load

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/modules/nostr-feed/services/SubmissionService.ts

@@ -148,6 +148,8 @@ export class SubmissionService extends BaseService {
         onEvent: (event: NostrEvent) => this.handleEvent(event),
         onEose: () => {
           this.debug('End of stored events')
+          // Refresh all submission votes from loaded reactions
+          this.refreshAllSubmissionVotes()
           this._isLoading.value = false
         },
         onClose: () => {
@@ -229,6 +231,8 @@ export class SubmissionService extends BaseService {
         onEvent: (event: NostrEvent) => this.handleEvent(event),
         onEose: () => {
           this.debug('End of stored events for submission')
+          // Refresh submission votes from loaded reactions
+          this.refreshSubmissionVotes(submissionId)
           // After loading comments, fetch their reactions
           this.fetchCommentReactions(submissionId)
           this._isLoading.value = false
@@ -907,12 +911,23 @@ export class SubmissionService extends BaseService {
    * Downvote a submission
    */
   async downvote(submissionId: string): Promise<void> {
-    // TODO: Implement downvote using '-' reaction content
-    // For now, this is a placeholder that mirrors the upvote logic
     const submission = this._submissions.get(submissionId)
     if (!submission) throw new Error('Submission not found')
 
-    this.debug('Downvote not yet implemented')
+    if (submission.votes.userVote === 'downvote') {
+      // Remove downvote
+      await this.reactionService?.undislikeEvent(submissionId)
+    } else {
+      // Add downvote (ReactionService keeps only latest reaction per user)
+      await this.reactionService?.dislikeEvent(
+        submissionId,
+        submission.pubkey,
+        SUBMISSION_KINDS.SUBMISSION
+      )
+    }
+
+    // Refresh votes
+    this.refreshSubmissionVotes(submissionId)
   }
 
   /**
@@ -926,6 +941,15 @@ export class SubmissionService extends BaseService {
     submission.ranking = this.calculateRanking(submission, submission.votes)
   }
 
+  /**
+   * Refresh votes for all submissions
+   */
+  private refreshAllSubmissionVotes(): void {
+    for (const submissionId of this._submissions.keys()) {
+      this.refreshSubmissionVotes(submissionId)
+    }
+  }
+
   /**
    * Upvote a comment
    */