fix: resolve Next.js warnings and SVG path errors

[michelroegl-brunner]

Summary

This PR resolves several Next.js warnings and fixes SVG path errors in the application.

Changes Made

🔧 Next.js Viewport Warning Fix

• File: src/app/layout.tsx
• Issue: Unsupported metadata viewport configuration warning
• Fix: Moved viewport configuration from metadata export to separate viewport export (Next.js 14+ compliance)

🎨 SVG Path Error Fix

• File: src/app/_components/CategorySidebar.tsx
• Issue: Malformed SVG arc command causing React DOM error
• Fix: Corrected arc command from A6 6 0 1721 9z to A6 6 0 1 0 21 9z (added missing arc flags)

🌐 Cross-Origin Request Warning Fix

• File: next.config.js
• Issue: Cross-origin request warning from local network devices
• Fix: Added allowedDevOrigins configuration supporting all private network ranges:
  • Localhost and loopback addresses
  • Class A private networks (10.x.x.x)
  • Class B private networks (172.16.x.x - 172.31.x.x)
  • Class C private networks (192.168.x.x)

Benefits

• ✅ Eliminates Next.js console warnings
• ✅ Fixes SVG rendering errors
• ✅ Enables access from any local network device without hardcoding IPs
• ✅ Maintains security by only allowing private network ranges
• ✅ Future-proof configuration that works with any user's network setup

Testing

• Verified viewport metadata warning is resolved
• Confirmed SVG path renders correctly without console errors
• Tested cross-origin access from local network devices