Implement modular Plasma application pinning system

- Add comprehensive Plasma taskbar configuration to users/jsutter.nix
- Configure pinned applications based on active desktop modules:
  * Base (plasma): konsole, dolphin, firefox, tigervnc, tor, kleopatra
  * Development (dev.nix): windsurf
  * Office (office.nix): slack, signal, libreoffice-writer, libreoffice-calc
  * Gaming (gaming.nix): discord, steam
- Add required packages to plasma.nix: firefox, tor-browser, kleopatra
- Use static list approach - Plasma ignores missing .desktop files
- Applications only appear when their corresponding module is active
- Maintains modularity while providing consistent desktop experience

Result: Clean, adaptive taskbar that automatically adjusts based on
which desktop modules are included in the system configuration.

desktop/plasma.nix

@@ -18,6 +18,9 @@
     arc-theme        # Desktop theme
     pinentry         # GUI pinentry for GPG
     tigervnc         # VNC client/server
+    firefox          # Web browser
+    tor-browser-bundle-bin  # Tor browser
+    kleopatra        # GPG key manager
   ];
 
   # GNUPG Stuff