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

home.nix

@@ -17,5 +17,31 @@
     enableFishIntegration = true;
   };
 
+  programs.alacritty = {
+    enable = true;
+    settings = {
+      window = {
+        padding = { x = 10; y = 10; };
+        opacity = 0.95;
+        decorations = "None";
+      };
+      font = {
+        normal = { family = "JetBrainsMono Nerd Font"; style = "Regular"; };
+        size = 11;
+      };
+      colors = {
+        primary = { background = "#1e1e2e"; foreground = "#cdd6f4"; };
+      };
+    };
+  };
+
+  xdg.configFile = {
+    "niri/config.kdl".source       = ./niri/config.kdl;
+    "waybar/config.jsonc".source   = ./niri/waybar/config.jsonc;
+    "waybar/style.css".source      = ./niri/waybar/style.css;
+    "mako/config".source           = ./niri/mako/config;
+    "fuzzel/fuzzel.ini".source     = ./niri/fuzzel/fuzzel.ini;
+  };
+
   programs.home-manager.enable = true;
 }