Add RTL-SDR support; switch to Limine + SDDM; drop Hyprland-era leftovers

RTL-SDR Blog V4. The V4 swapped the R820T2 tuner for an R828D behind a
built-in HF upconverter, so it needs the rtlsdrblog fork of librtlsdr --
the stock Osmocom one reports no supported devices. pkgs.rtl-sdr on this
channel already is that fork (rtl-sdr-blog 1.3.5), so hardware.rtl-sdr's
default package is correct as-is. That also blacklists the DVB-T drivers
that otherwise grab the dongle on plug-in, and ships udev rules handing it
to plugdev at 0660 -- hence plugdev in the user's groups, or every SDR app
needs root. Apps: SDR++ and rtl_433; the rtl_* CLI tools come with the
driver.

Both SDR apps are overridden to drop hardware we do not own. SDR++ and
rtl_433 default to supporting every radio ever made, which pulled in UHD
(1.4 GB of USRP firmware), BladeRF and LimeSuite. gqrx was the original
choice but reaches UHD through gr-osmosdr's gr-uhd feature; SDR++ uses no
GNU Radio at all. Per-package overrides rather than a global overlay, so
nothing else is affected.

Bootloader: systemd-boot -> Limine, themed to match the Gruvbox icon
theme. Limine installs to its own EFI directory and adds its own NVRAM
entry, so systemd-boot stays on the ESP as a firmware-menu fallback.

Display manager: lightdm -> SDDM with wayland.enable. Budgie in this
channel ships only a Wayland session (labwc) and was already running as
one; lightdm just handles that badly. SDDM's greeter runs on weston, not
kwin, so no KDE compositor comes along.

Qt defaults over Budgie's GTK/MATE apps. The Budgie module force-enables
gnome-terminal and installs nemo, eom, pluma, atril, engrampa, mate-calc
and mate-system-monitor; environment.budgie.excludePackages drops all of
them in favour of konsole, dolphin, gwenview, kate, okular, ark, kcalc and
plasma-systemmonitor. default-apps.nix rewritten to match: 113
associations, video to vlc, audio to elisa, text/code to kate, and
/etc/xdg/xdg-terminals.list pointing at konsole. Fixes a real bug there --
inode/directory was mapped to gwenview, so folders opened in the image
viewer.

Hyprland-era leftovers removed. system-telemetry and weather-fetch fed
Quickshell panels that no longer exist; brightness-ctl and hotspot-start
lost their keybinds. jq went with weather-fetch. Also dropped
gnome-screenshot, playerctl and imv. vault-toggle and strip-metadata stay
-- they never depended on Hyprland.

Comments trimmed throughout: deleted anything restating the code, kept the
ones recording a why you cannot recover by reading it (systemd.gpt_auto,
disable_ertm, ptrace_scope, the stable ethernet MAC, the mimeapps.list
parser gotcha). system.nix went 225 -> 166 lines.

README rewritten -- it still described Hyprland, Quickshell and Kitty.
Screenshot dropped.

modules/drivers.nix

@@ -1,43 +1,34 @@
 { config, lib, pkgs, ... }:
 
 {
-  # Enable graphics driver support and 32-bit acceleration (for Steam/gaming)
   hardware.graphics = {
     enable = true;
     enable32Bit = true;
   };
 
-  #OpenTabletDriver
   hardware.opentabletdriver.enable = true;
   hardware.uinput.enable = true;
   boot.kernelModules = [ "uinput" ];
 
   boot.extraModprobeConfig = ''
-    # PS4/PS5 controllers (e.g. DualSense) connect over Bluetooth then
-    # immediately disconnect — a well-known Linux Bluetooth stack issue
-    # where many adapters' Enhanced Retransmission Mode implementation is
-    # broken. Disabling ERTM is the standard, widely-documented fix.
+    # DualSense/PS4 pads pair over Bluetooth then drop instantly; broken
+    # ERTM in many adapters. Disabling it is the standard fix.
     options bluetooth disable_ertm=1
   '';
 
-  # I2C bus, for DDC/CI monitor control (brightness) via ddcutil — this
-  # machine has no laptop backlight, only external DisplayPort monitors.
+  # Blog V4 needs the rtlsdrblog fork of librtlsdr, which is what
+  # pkgs.rtl-sdr already is here. Also blacklists the DVB-T drivers.
+  hardware.rtl-sdr.enable = true;
+
+  # For ddcutil — no laptop backlight here, only external DisplayPort.
   hardware.i2c.enable = true;
 
-  # Load NVIDIA driver for X11 and Wayland
   services.xserver.videoDrivers = [ "nvidia" ];
 
   hardware.nvidia = {
-    # Required settings
     modesetting.enable = true;
-
-    # Enable NVIDIA Open GPU Kernel Modules (Turing architecture or newer)
     open = true;
-
-    # Enable the NVIDIA settings menu
     nvidiaSettings = true;
-
-    # Use the stable driver package matching current kernel
     package = config.boot.kernelPackages.nvidiaPackages.stable;
   };
 }