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/default.nix

@@ -23,6 +23,8 @@
     ./shell.nix     # zsh + starship + bat/eza/zoxide, themed
     ./keys.nix      # gpg-agent (fronts SSH) + pinentry-qt
     ./fastfetch.nix # system info with the themed Nomarchy logo
+    ./viewers.nix   # zathura (Stylix-themed) + imv — PDF/image viewing
+    ./mime.nix      # default applications (mimeapps.list), degrades with the suite
   ];
 
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