Fix KDE package references for Qt6 migration

- Update kdenlive to kdePackages.kdenlive
- Update okular to kdePackages.okular
- Resolves libsForQt5 deprecation warnings

🤖 Generated with [Claude Code](https://claude.ai/code)

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

modules/packages.nix

@@ -222,7 +222,7 @@ in
         mpv
         vlc
         obs-studio
-        kdenlive
+        kdePackages.kdenlive
         handbrake
         ffmpeg-full
 
@@ -245,7 +245,7 @@ in
         # PDF
         zathura
         evince
-        okular
+        kdePackages.okular
         mupdf
       ]