fix: always show zap count on post cards, including 0

[barrydeen]

Summary

• Post cards previously hid the zap amount entirely when a note had no zaps, leaving a blank space next to the bolt icon.
• Now the count always renders (showing 0 when there are no zaps), matching how like and repost counts behave.

Test plan

• Open the feed and confirm posts with no zaps show 0 next to the bolt icon
• Confirm posts with zaps still display the formatted total (e.g. 1.2k)
• Confirm the count is hidden while a zap is in progress (loading state)