terra: flatpak, unstable packages, GTK dark theme, comms workspace

- nix-flatpak input; discord, telegram, qbz as Flathub flatpaks; removes
  qbz and proton-pass-cli flake inputs
- proton-pass-cli and claude-code from nixpkgs-unstable via extraSpecialArgs
- dconf color-scheme = prefer-dark replaces per-session gsettings call

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/terra/configuration.nix

@@ -5,6 +5,12 @@
 # /mnt/hdd_01, LVM vg_ssd /mnt/ssd_01) are kept out of disko and mounted here
 # as plain filesystems so they're never wiped. The leftover ntfs disks
 # (sda, sdf, nvme0n1) are ignored entirely — not referenced anywhere.
+let
+  unstable = import inputs.nixpkgs-unstable {
+    inherit (pkgs.stdenv.hostPlatform) system;
+    config = pkgs.config;
+  };
+in
 {
   imports = [
     ./hardware-configuration.nix
@@ -28,15 +34,15 @@
     ];
   };
 
-  # Proton Pass CLI — not in nixpkgs; ./scripts/deploy uses it to autofill
-  # sudo/ssh passwords from the "HomeLab" vault. Packaged by the
-  # proton-pass-cli flake input (github:tomsch/proton-pass-cli-nix).
-  environment.systemPackages = [ inputs.proton-pass-cli.packages.${pkgs.system}.default ];
+  # Proton Pass CLI — in nixpkgs-unstable (2.2.3+); scripts/deploy uses it to
+  # autofill sudo/ssh passwords from the "HomeLab" vault.
+  environment.systemPackages = [ unstable.proton-pass-cli ];
 
   # ---- home-manager (user-level config for darman) ----
   home-manager.useGlobalPkgs = true;
   home-manager.useUserPackages = true;
   home-manager.backupFileExtension = "hm-bak";
+  home-manager.extraSpecialArgs = { inherit unstable; };
   home-manager.users.darman = import ./home.nix;
 
   # ---- Boot (UEFI) ----