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/mako/config

@@ -0,0 +1,20 @@
+font=JetBrainsMono Nerd Font 10
+background-color=#1e1e2eee
+text-color=#cdd6f4
+border-color=#89b4fa
+border-size=2
+border-radius=8
+padding=12
+margin=8
+default-timeout=5000
+ignore-timeout=1
+max-visible=5
+layer=overlay
+anchor=top-right
+
+[urgency=low]
+border-color=#6c7086
+
+[urgency=high]
+border-color=#f38ba8
+default-timeout=0