feat(media): add zathura pdf/document viewer

minimal keyboard-driven viewer; bundled mupdf backend covers PDFs out of the
box. yank selection to the wayland clipboard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/media.nix

@@ -1,5 +1,5 @@
-# Generic media viewers. mpv for video/audio, imv for images — both minimal,
-# keyboard-driven, Wayland-native. No personal config here.
+# Generic viewers. mpv for video/audio, imv for images, zathura for PDFs/docs —
+# all minimal, keyboard-driven, Wayland-native. No personal config here.
 #
 # imv key binds run external actions on the current file (yank path, rotate/flip,
 # delete, open in editor, info). Wallpaper uses awww (daemon + CLI): `setbg`
@@ -41,6 +41,13 @@ in
     };
   };
 
+  # PDF/document viewer. pkgs.zathura bundles the mupdf backend, so enabling is
+  # enough. Route yanks to the wayland clipboard.
+  programs.zathura = {
+    enable = true;
+    options.selection-clipboard = "clipboard";
+  };
+
   home.packages = with pkgs; [
     setbg
     awww            # wayland wallpaper daemon (setbg backend)