niri: add full desktop stack (bar, launcher, notifications, configs)

Moves programs.niri into a dedicated niri.nix module and installs the
usual Wayland userland — alacritty, fuzzel, waybar, mako, swaybg,
grim/slurp/wl-clipboard, swaylock/swayidle/wlogout, xwayland-satellite,
plus nm-applet/pavucontrol/playerctl/brightnessctl.

Ships starter configs for niri (keybinds mirroring sway/KDE habits,
screenshots to ~/Pictures/Screenshots, media keys, workspace switching,
auto-lock via swayidle), waybar with a Catppuccin-ish style, mako, and
fuzzel — all wired up through home-manager's xdg.configFile.

https://claude.ai/code/session_01NrGdGEBRbDbWvHUb3Lho44

niri.nix

@@ -0,0 +1,43 @@
+{ pkgs, ... }:
+
+{
+  programs.niri.enable = true;
+
+  environment.systemPackages = with pkgs; [
+    # terminal + launcher
+    alacritty
+    fuzzel
+
+    # bar, notifications, wallpaper
+    waybar
+    mako
+    swaybg
+
+    # screenshots + clipboard
+    grim
+    slurp
+    swappy
+    wl-clipboard
+
+    # lock + idle
+    swaylock-effects
+    swayidle
+    wlogout
+
+    # XWayland shim for niri
+    xwayland-satellite
+
+    # tray + system utilities
+    networkmanagerapplet
+    pavucontrol
+    playerctl
+    brightnessctl
+    libnotify
+    xdg-utils
+  ];
+
+  environment.sessionVariables = {
+    XDG_CURRENT_DESKTOP = "niri";
+    XDG_SESSION_DESKTOP = "niri";
+  };
+}