Switch desktop environment from Hyprland to Budgie; app/audio overhaul

Hyprland problems prompted a full switch to Budgie (X11, lightdm).
Removes every Hyprland/Quickshell dotfile, theming script, and
hypr-specific module (theme.nix, the whole scripts/ theming pipeline,
hyprland.conf/hyprpaper.conf, the Quickshell shell QML, kitty/mako/
rofi configs) in favor of Budgie's own panel/session/theming and a
GSettings-driven icon/cursor override.

App changes:
- Konsole/Dolphin replace kitty/Thunar as the default terminal and
  file manager; dropped xterm too (services.xserver.excludePackages)
- Added Okular (PDF/document viewer), KCalc (calculator), Elisa
  (audio player) -- nothing filled those roles before
- Added OBS Studio

Audio/voice:
- EasyEffects for real-time mic/output effects (creates its own
  selectable "Easy Effects Source" virtual mic automatically)
- A standalone "Virtual Microphone" PipeWire device (loopback module,
  services.pipewire.extraConfig), fully independent of the real mic,
  for routing EasyEffects/Soundux output separately from what Discord
  etc. use by default
- qpwgraph for manual patchbay routing into the above
- Soundux (soundboard) installed via Flatpak -- no longer packaged in
  this nixpkgs channel

default-apps.nix: Okular wired into mimeapps.list alongside the
existing Gwenview/Ark defaults; removed the Thunar-specific
"Open Terminal Here" custom-action tmpfiles rule (dead now that
Thunar's gone -- Dolphin gets the same via its built-in konsolepart
terminal panel for free).

configuration.nix

@@ -7,8 +7,7 @@
     ./hardware-configuration.nix   # Generated hardware scan results
     ./modules/system.nix           # Bootloader, networking, time, sound, hardening, nix-ld
     ./modules/drivers.nix          # NVIDIA Open GPU kernel modules and graphics
-    ./modules/desktop.nix          # Hyprland compositor and desktop environment
-    ./modules/theme.nix            # Shared color palette; GTK dark+squared theming
+    ./modules/desktop.nix          # Budgie desktop environment
     ./modules/users.nix            # User accounts and global configurations
     ./modules/apps.nix             # All installed packages: apps, CLI tools, dev toolchains
     ./modules/scripts.nix          # Custom system scripts and helpers