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/style.css

@@ -0,0 +1,62 @@
+* {
+  font-family: "JetBrainsMono Nerd Font", "Geist", sans-serif;
+  font-size: 13px;
+  border: none;
+  border-radius: 0;
+  min-height: 0;
+}
+
+window#waybar {
+  background: rgba(30, 30, 46, 0.85);
+  color: #cdd6f4;
+  border-radius: 10px;
+}
+
+#workspaces button {
+  padding: 0 8px;
+  color: #9399b2;
+  background: transparent;
+  border-radius: 6px;
+}
+
+#workspaces button.active {
+  color: #1e1e2e;
+  background: #89b4fa;
+}
+
+#workspaces button.urgent {
+  background: #f38ba8;
+  color: #1e1e2e;
+}
+
+#window,
+#clock,
+#cpu,
+#memory,
+#network,
+#pulseaudio,
+#tray,
+#custom-power {
+  padding: 0 10px;
+  margin: 4px 2px;
+  border-radius: 6px;
+  background: rgba(49, 50, 68, 0.6);
+}
+
+#clock {
+  color: #f9e2af;
+  font-weight: 600;
+}
+
+#custom-power {
+  color: #f38ba8;
+  padding: 0 12px;
+}
+
+#network.disconnected {
+  color: #f38ba8;
+}
+
+#pulseaudio.muted {
+  color: #6c7086;
+}