Fix KDE systemsettings default applications menu issue

KDE applications in i3 were showing only "Other..." in default
applications due to missing applications.menu file. Plasma 6 renamed
applications.menu to plasma-applications.menu but KDE components
still look for the old name.

This creates a symlink from the new location to the expected location
using XDG config file management.

Fixes kmail and systemsettings not detecting installed applications.

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

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

home/roles/desktop/default.nix

@@ -19,9 +19,11 @@ in
       kitty
       
       # Desktop utilities
+      feh # Image viewer and wallpaper setter for X11
       solaar # Logitech management software
       waybar
       wofi
+      xdg-utils # XDG utilities for opening files/URLs with default applications
       
       # System utilities with GUI components
       (snapcast.override { pulseaudioSupport = true; })
@@ -37,6 +39,14 @@ in
       kdePackages.kaddressbook
       kdePackages.kontact
       
+      # KDE System components needed for proper integration
+      kdePackages.kded
+      kdePackages.systemsettings
+      kdePackages.kmenuedit
+      
+      # Desktop menu support
+      kdePackages.plasma-desktop # Contains applications.menu
+      
       # KDE Online Accounts support
       kdePackages.kaccounts-integration
       kdePackages.kaccounts-providers
@@ -70,7 +80,79 @@ in
       enable = true;
     };
 
-    xdg.enable = true;
+    # KDE environment variables for proper integration
+    home.sessionVariables = {
+      QT_QPA_PLATFORMTHEME = "kde";
+      KDE_SESSION_VERSION = "6";
+    };
+
+    xdg = {
+      enable = true;
+      
+      # Ensure desktop files are made available for discovery
+      desktopEntries = {};  # This creates the desktop files directory structure
+      
+      mimeApps = {
+        enable = true;
+        associations.added = {
+          # Ensure associations are properly registered
+          "text/html" = "firefox.desktop";
+          "x-scheme-handler/http" = "firefox.desktop";
+          "x-scheme-handler/https" = "firefox.desktop";
+        };
+        defaultApplications = {
+          # Web browsers
+          "text/html" = "firefox.desktop";
+          "x-scheme-handler/http" = "firefox.desktop";
+          "x-scheme-handler/https" = "firefox.desktop";
+          "x-scheme-handler/about" = "firefox.desktop";
+          "x-scheme-handler/unknown" = "firefox.desktop";
+          
+          # Documents
+          "application/pdf" = "okular.desktop";
+          "text/plain" = "kate.desktop";
+          "text/x-tex" = "kate.desktop";
+          "text/x-c" = "kate.desktop";
+          "text/x-python" = "kate.desktop";
+          "application/x-shellscript" = "kate.desktop";
+          
+          # Images
+          "image/png" = "gwenview.desktop";
+          "image/jpeg" = "gwenview.desktop";
+          "image/jpg" = "gwenview.desktop";
+          "image/gif" = "gwenview.desktop";
+          "image/bmp" = "gwenview.desktop";
+          "image/tiff" = "gwenview.desktop";
+          "image/webp" = "gwenview.desktop";
+          
+          # Archives
+          "application/zip" = "ark.desktop";
+          "application/x-tar" = "ark.desktop";
+          "application/x-compressed-tar" = "ark.desktop";
+          "application/x-7z-compressed" = "ark.desktop";
+          "application/x-rar" = "ark.desktop";
+          
+          # Audio
+          "audio/mpeg" = "elisa.desktop";
+          "audio/mp4" = "elisa.desktop";
+          "audio/flac" = "elisa.desktop";
+          "audio/ogg" = "elisa.desktop";
+          "audio/wav" = "elisa.desktop";
+          
+          # Email
+          "message/rfc822" = "kmail.desktop";
+          "x-scheme-handler/mailto" = "kmail.desktop";
+          
+          # Calendar
+          "text/calendar" = "korganizer.desktop";
+          "application/x-vnd.akonadi.calendar.event" = "korganizer.desktop";
+        };
+      };
+    };
+
+    # Fix for KDE applications.menu file issue on Plasma 6
+    # KDE still looks for applications.menu but Plasma 6 renamed it to plasma-applications.menu
+    xdg.configFile."menus/applications.menu".source = "${pkgs.kdePackages.plasma-workspace}/etc/xdg/menus/plasma-applications.menu";
 
     # Note: modules must be imported at top-level home config
   };