chore(backup): replace emoji codes with actual Unicode emojis

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_backup/backup.sh

@@ -60,7 +60,7 @@ fi
 log "Running restic backup"
 if ! restic backup "$DATA_DIR" --tag stacks 2>&1 | tee -a "$LOG_FILE"; then
   log "FATAL: restic backup failed"
-  notify "#cc0000" ":x: **Backup failed** — restic backup error. Check \`$LOG_FILE\`."
+  notify "#cc0000" "❌ **Backup failed** — restic backup error. Check \`$LOG_FILE\`."
   exit 1
 fi
 
@@ -74,8 +74,8 @@ fi
 snapshot_info=$(restic snapshots --latest 1 --compact 2>/dev/null | tail -3 | head -1)
 repo_stats=$(restic stats 2>/dev/null | grep "Total Size" || true)
 
-summary=":white_check_mark: **Backup complete**"
-[ ${#dump_errors[@]} -gt 0 ] && summary+="\n:warning: Failed dumps: ${dump_errors[*]}"
+summary="✅ **Backup complete**"
+[ ${#dump_errors[@]} -gt 0 ] && summary+="\n⚠️ Failed dumps: ${dump_errors[*]}"
 summary+="\nLatest: \`$snapshot_info\`"
 [ -n "$repo_stats" ] && summary+="\nRepo: $repo_stats"