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/waybar/config.jsonc

@@ -0,0 +1,77 @@
+{
+  "layer": "top",
+  "position": "top",
+  "height": 30,
+  "spacing": 6,
+  "margin-top": 4,
+  "margin-left": 8,
+  "margin-right": 8,
+
+  "modules-left": ["niri/workspaces", "niri/window"],
+  "modules-center": ["clock"],
+  "modules-right": [
+    "tray",
+    "network",
+    "pulseaudio",
+    "cpu",
+    "memory",
+    "custom/power"
+  ],
+
+  "niri/workspaces": {
+    "format": "{icon}",
+    "format-icons": {
+      "default": "",
+      "active": "",
+      "urgent": ""
+    }
+  },
+
+  "niri/window": {
+    "format": "{title}",
+    "max-length": 80,
+    "separate-outputs": true
+  },
+
+  "clock": {
+    "format": "{:%a %d %b  %H:%M}",
+    "tooltip-format": "<tt><small>{calendar}</small></tt>"
+  },
+
+  "cpu": {
+    "format": " {usage}%",
+    "interval": 2
+  },
+
+  "memory": {
+    "format": " {used:0.1f}G",
+    "interval": 5
+  },
+
+  "network": {
+    "format-wifi": "  {essid}",
+    "format-ethernet": " {ifname}",
+    "format-disconnected": "⚠ offline",
+    "tooltip-format": "{ifname} · {ipaddr}"
+  },
+
+  "pulseaudio": {
+    "format": "{icon} {volume}%",
+    "format-muted": " muted",
+    "format-icons": {
+      "default": ["", "", ""]
+    },
+    "on-click": "pavucontrol"
+  },
+
+  "tray": {
+    "icon-size": 16,
+    "spacing": 8
+  },
+
+  "custom/power": {
+    "format": "⏻",
+    "tooltip": false,
+    "on-click": "wlogout"
+  }
+}