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/default-apps.nix

@@ -2,27 +2,26 @@
 
 {
   environment.systemPackages = with pkgs; [
-    libreoffice-qt6 # Qt6-styled build — fits the Qt theming work in theme.nix
-                     # better than the default GTK3 build would.
+    libreoffice-qt6
   ];
 
-  # System-level fallback defaults (XDG spec: only consulted for a
-  # mimetype the user's own ~/.config/mimeapps.list doesn't already
-  # claim), so a future "Open With" choice always wins over this without
-  # needing to touch it. Mimetype lists taken directly from Gwenview's
-  # and Ark's own .desktop files, not guessed.
+  # GIO picks the terminal from here for "Open Terminal" style actions.
+  environment.etc."xdg/xdg-terminals.list".text = ''
+    org.kde.konsole.desktop
+  '';
+
+  # System-level fallbacks. Per the XDG spec these are only consulted for a
+  # mimetype the user's own ~/.config/mimeapps.list doesn't already claim,
+  # so any "Open With" choice you make later still wins over this.
   #
-  # One mimetype=desktopfile pair per line: the semicolon only separates
-  # multiple desktop-file candidates for the SAME key, it doesn't chain
-  # separate mimetype=value pairs on one line — mimeapps.list parsers
-  # (verified against xdg-utils' own xdg-mime script) match by scanning
-  # for a line that STARTS WITH "mimetype=", so cramming several pairs
-  # onto one semicolon-joined line silently hides every key after the
-  # first from ever being found.
+  # One mimetype=desktopfile pair per line. The semicolon only separates
+  # multiple candidates for the SAME key — it does not chain separate pairs,
+  # and parsers match by scanning for a line starting with "mimetype=", so
+  # cramming several onto one line silently hides all but the first.
   environment.etc."xdg/mimeapps.list".text =
     let
       gwenviewMimes = [
-        "inode/directory" "image/avif" "image/gif" "image/heif" "image/jpeg"
+        "image/avif" "image/gif" "image/heif" "image/jpeg"
         "image/jxl" "image/png" "image/bmp" "image/x-eps" "image/x-icns"
         "image/x-ico" "image/x-portable-bitmap" "image/x-portable-graymap"
         "image/x-portable-pixmap" "image/x-xbitmap" "image/x-xpixmap"
@@ -42,19 +41,40 @@
         "application/x-7z-compressed" "application/vnd.rar" "application/zip"
         "application/x-java-archive" "application/x-arj"
       ];
-      # Taken from Okular's own .desktop file, same as the other two lists.
       okularMimes = [
         "application/pdf" "application/x-pdf" "image/vnd.djvu" "application/epub+zip"
         "application/x-fictionbook+xml" "application/x-mobipocket-ebook"
         "image/x-portable-document" "application/x-cbr" "application/x-cbz"
         "application/x-cbt" "application/x-cb7" "application/postscript"
       ];
+      vlcMimes = [
+        "video/mp4" "video/x-matroska" "video/webm" "video/x-msvideo"
+        "video/quicktime" "video/mpeg" "video/x-ms-wmv" "video/x-flv"
+        "video/3gpp" "video/ogg" "video/x-theora+ogg" "video/x-ogm+ogg"
+        "application/x-matroska" "application/vnd.rn-realmedia"
+      ];
+      elisaMimes = [
+        "audio/mpeg" "audio/flac" "audio/x-flac" "audio/ogg" "audio/x-vorbis+ogg"
+        "audio/opus" "audio/x-opus+ogg" "audio/mp4" "audio/aac" "audio/x-wav"
+        "audio/wav" "audio/x-ms-wma" "audio/x-aiff" "audio/x-musepack"
+      ];
+      kateMimes = [
+        "text/plain" "text/markdown" "text/x-csrc" "text/x-c++src" "text/x-chdr"
+        "text/x-c++hdr" "text/x-python" "text/x-java" "text/x-shellscript"
+        "text/x-nix" "text/css" "text/html" "text/xml" "application/json"
+        "application/x-yaml" "application/toml" "application/javascript"
+        "application/x-shellscript" "text/x-log"
+      ];
       line = desktop: mime: "${mime}=${desktop}";
     in
     ''
       [Default Applications]
+      inode/directory=org.kde.dolphin.desktop
       ${lib.concatMapStringsSep "\n" (line "org.kde.gwenview.desktop") gwenviewMimes}
       ${lib.concatMapStringsSep "\n" (line "org.kde.ark.desktop") arkMimes}
       ${lib.concatMapStringsSep "\n" (line "org.kde.okular.desktop") okularMimes}
+      ${lib.concatMapStringsSep "\n" (line "vlc.desktop") vlcMimes}
+      ${lib.concatMapStringsSep "\n" (line "org.kde.elisa.desktop") elisaMimes}
+      ${lib.concatMapStringsSep "\n" (line "org.kde.kate.desktop") kateMimes}
     '';
 }