Bring laptop in line with desktop config: Plasma 6, Limine, module split

The laptop was still on the Hyprland-era Pantheon commit while the desktop
moved to KDE Plasma 6 + Limine and split system.nix into focused modules.
This brings over everything that isn't hardware, unchanged, so both machines
stay diffable:

- Plasma 6 on Wayland with SDDM, Konsole, gruvbox icons, KDE MIME defaults
  (replaces Pantheon/LightDM; libreoffice-qt6 replaces the GTK build)
- Limine bootloader with the gruvbox theme, replacing systemd-boot
- system.nix split into boot/networking/locale/nix-settings/security
- flakes + nix-command, sysctl hardening extras (yama, tcp_timestamps),
  geoclue off, captive-portal probe off
- package parity: obs-studio, lmms, go, qemu, KDE utilities, lean SDR++ and
  rtl_433 overrides with hardware.rtl-sdr

Hardware and power are adapted rather than copied:

- hardware.nix keeps the AMD 680M + RTX 4050 PRIME offload setup, asusd and
  supergfxd, v4l2loopback webcam and the DualSense ERTM fix; OpenRGB and
  the desktop's single-GPU block are dropped, keyboard lighting is asusctl
- power.nix is new: power-profiles-daemon for Plasma's profile switcher,
  lid-close handling via the current logind.settings.Login spelling,
  powertop/acpi, notes on asusctl charge limits
- no acpi_enforce_resources=lax, ACPI owns the EC asusd drives here
- touchpad defaults, and firewall openings so the hotspot script's shared
  connection can hand out DHCP/DNS
- pinentry-qt instead of the desktop's leftover pinentry-rofi, blueman
  dropped in favour of Plasma's bluedevil

hardware-configuration.nix stays tracked here (unlike the desktop, which
gitignores it) since it's the only record of this machine's disk layout.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

modules/boot.nix

@@ -0,0 +1,47 @@
+{ config, lib, pkgs, ... }:
+
+{
+  boot.loader.efi.canTouchEfiVariables = true;
+  boot.loader.limine = {
+    enable = true;
+    maxGenerations = 5;
+
+    # Gruvbox, to match the icon theme.
+    style = {
+      wallpapers = [ ];   # Solid background; a path here would be used instead
+      interface = {
+        branding = "NixOS";
+        brandingColor = "fabd2f";
+        helpColor = "928374";
+        helpColorBright = "d79921";
+      };
+      graphicalTerminal = {
+        foreground = "ebdbb2";
+        background = "00282828";
+        brightForeground = "fbf1c7";
+        palette = "282828;cc241d;98971a;d79921;458588;b16286;689d6a;a89984";
+        brightPalette = "928374;fb4934;b8bb26;fabd2f;83a598;d3869b;8ec07c;ebdbb2";
+        margin = 32;
+      };
+    };
+  };
+
+  boot.kernelPackages = pkgs.linuxPackages;
+  boot.supportedFilesystems = [ "fuse" ];
+  programs.fuse.userAllowOther = true;
+
+  # zram instead of disk swap: nothing sensitive is written to disk in
+  # plaintext. systemd's gpt-auto-generator would otherwise re-activate the
+  # swap partition hardware-configuration.nix still lists, hence the kernel
+  # param. / /home /nix /boot are declared explicitly there, so they don't
+  # depend on gpt-auto.
+  zramSwap.enable = true;
+  swapDevices = lib.mkForce [ ];
+
+  # Unlike the desktop this doesn't carry acpi_enforce_resources=lax: that
+  # only exists there to let lm_sensors poke the Super I/O chip behind ACPI's
+  # back, and on a laptop — where ACPI genuinely owns the EC that asusd talks
+  # to — relaxing it is how you get fan/thermal weirdness. CPU temps come
+  # from k10temp, everything else from asusd.
+  boot.kernelParams = [ "systemd.gpt_auto=0" ];
+}