feat(desktop): viewers (zathura+imv) + xdg-mime defaults (item 8)

The complete-workstation gap: no PDF/image viewer shipped and nothing
set mime defaults, so 'open a photo' fell to GIMP. Two new default-on
toggles: nomarchy.viewers (programs.zathura — Stylix themes it, target
added to the explicit list — plus imv) and nomarchy.mime (mkDefault
xdg.mimeApps: pdf→zathura, image→imv, av→mpv, text→code, dir→thunar;
html/http(s)→firefox as inert entries that only activate when a
browser is installed — the ship-a-browser Decision stays open, now
smaller). Associations for absent apps are skipped by GIO, so the
whole thing degrades as the user curates the suite. Deviation from
the backlog text: viewers are a module toggle, not template package
lines — zathura's theming needs its HM module (the real-config
boundary of the toggle discipline).

V1: template-home generation asserted — mimeapps.list rendered, themed
zathurarc, binaries in home-path. V3 open-a-file smoke queued.

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

modules/home/viewers.nix

@@ -0,0 +1,15 @@
+# Document & image viewers — the "open a PDF / open a photo" half of a
+# complete workstation. zathura goes through its HM module (not a bare
+# package) because Stylix themes it via programs.zathura.options — the
+# reason it's a component toggle here rather than a template package
+# line. imv rides along: wayland-native, and an image viewer is a
+# borderless dark surface — nothing to theme. The heavier editors
+# (GIMP, Inkscape) stay template packages; mime.nix points the default
+# associations at these viewers.
+{ config, lib, pkgs, ... }:
+
+lib.mkIf config.nomarchy.viewers.enable {
+  programs.zathura.enable = lib.mkDefault true;
+
+  home.packages = [ pkgs.imv ];
+}