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/fuzzel/fuzzel.ini

@@ -0,0 +1,23 @@
+font=JetBrainsMono Nerd Font:size=12
+dpi-aware=yes
+terminal=alacritty
+prompt="  "
+icon-theme=Adwaita
+width=40
+lines=12
+horizontal-pad=20
+vertical-pad=14
+inner-pad=10
+
+[colors]
+background=1e1e2eee
+text=cdd6f4ff
+match=f9e2afff
+selection=89b4faff
+selection-text=1e1e2eff
+selection-match=1e1e2eff
+border=89b4faff
+
+[border]
+width=2
+radius=10