Add About / Support page (roadmap #4)

- New 'about' view wired into sidebar nav (♥ support) and App.tsx
- Sections: in-app zap button (reuses ZapModal + NWC infra), Lightning
  address QR + copy, Bitcoin address QR + copy, Ko-fi link, GitHub link,
  version/tech credits footer
- QR codes rendered as SVG via react-qr-code (no canvas dependency)
- lightning: URI and bitcoin: URI formats for wallet-scannable QR codes
- Tasteful and non-nagging — lives in the nav, never pops up unprompted

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

package.json

@@ -17,6 +17,7 @@
     "marked": "^17.0.4",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
+    "react-qr-code": "^2.0.18",
     "tailwindcss": "^4.2.1",
     "zustand": "^5.0.11"
   },