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/scripts.nix

@@ -1,25 +1,14 @@
 { config, pkgs, ... }:
 
 let
-  # Wrap external shell scripts into system executables
   vaultToggle = pkgs.writeShellScriptBin "vault-toggle" (builtins.readFile ./scripts/vault-toggle.sh);
   stripMetadata = pkgs.writeShellScriptBin "strip-metadata" (builtins.readFile ./scripts/strip-metadata.sh);
-  brightnessCtl = pkgs.writeShellScriptBin "brightness-ctl" (builtins.readFile ./scripts/brightness-ctl.sh);
-  hotspotStart = pkgs.writeShellScriptBin "hotspot-start" (builtins.readFile ./scripts/hotspot-start.sh);
-  systemTelemetry = pkgs.writeShellScriptBin "system-telemetry" (builtins.readFile ./scripts/system-telemetry.sh);
-  weatherFetch = pkgs.writeShellScriptBin "weather-fetch" (builtins.readFile ./scripts/weather-fetch.sh);
 in
 {
-  # Install CLI dependencies required by the scripts
   environment.systemPackages = with pkgs; [
     gocryptfs
     exiftool
-    jq
     vaultToggle
     stripMetadata
-    brightnessCtl
-    hotspotStart
-    systemTelemetry
-    weatherFetch
   ];
 }