docs(readme): add the social card and refresh the app screenshot

[kNoAPP]

Description

Updates the top of the README to use the new brand artwork and show the current app.

• Social card: the README now opens with public/brand/social-card.png (from feat(brand): add original brand artwork #458) as a full-width banner. It uses the committed file, so it stays in sync with the brand folder.
• Screenshot: the old July screenshot showed the app under its previous name ("MeshCore Companion") and without the Nodes tab. It is replaced with a fresh 3200×1920 (2×) capture of the Chat view in the dark theme, hosted on user-attachments like before. The contacts, channels and messages in it are made-up sample data, not a real mesh.

Before	After

Issues

None

Type of Change

• ❌ ! -- Breaking change (fix or feature that would cause existing
  functionality to change)
• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• 📝 docs -- Additional or updated documentation
• 🎨 style -- Code style changes (formatting, missing semi colons, etc)
• 🧹 refactor -- Code refactor
• ⚡ perf -- Performance improvement
• 🧪 test -- Tests (adding or updating tests)
• 🏗️ build -- Build system or external dependencies changes
• ✅ ci -- Build configuration change
• 🗑️ chore -- Grunt tasks
• ⏪ revert -- Revert previous changes