Design: SocialMediaPosting tweet card layout

[melvincarvalho]

Design PR — edits app.js + app.css only (no app files touched).

• SocialMediaPosting gets a tweet card layout (class .card.tweet-card)
  • Header row: avatar circle with author initial, author name + pseudo-handle, icon badge
  • Post body as large tweet-style text (pre-wrap)
  • Inline media image with rounded corners
  • Timestamp in muted grey
  • Action icons row: 💬 Reply, 🔄 Repost, ❤️ Like,📤 Share (decorative, non-functional)
  • Dark mode support
  • Clean, minimal — no hero image, no generic rows

Gate: 324 apps pass. Task: #IssDesign2_SocialMediaPosting.