Fix DPI sizing issues

[Marc-André Moreau (mamoreau-devolutions)]

Summary

Fix issues found with DPI-scaling, it's more balanced now:

Fixes included

• Make saved window geometry DPI-aware so window size restores correctly across mixed-DPI monitors.
• Use DIP-based width checks instead of AppWindow pixel width for layout-sensitive UI decisions.
• Reduce the oversized discover-page mega search box.
• Tighten oversized headers and navigation item sizing that were disproportionately large at higher display scale factors.