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/options.nix

@@ -252,6 +252,8 @@ in
     btop.enable = lib.mkEnableOption "btop with the per-theme nomarchy theme" // { default = true; };
     stylix.enable = lib.mkEnableOption "Stylix theming for the long tail of apps (GTK, Qt, cursors)" // { default = true; };
     displays.enable = lib.mkEnableOption "the nwg-displays interactive monitor arranger (a helper to find nomarchy.monitors values; the declarative config stays the source of truth)" // { default = true; };
+    viewers.enable = lib.mkEnableOption "the document/image viewers (zathura, Stylix-themed, + imv)" // { default = true; };
+    mime.enable = lib.mkEnableOption "default file associations (xdg mimeapps.list: PDF/image/video/text/browser/directory); entries for absent apps are skipped, so it degrades with the package suite" // { default = true; };
 
     # ── Computed (read-only) ───────────────────────────────────────
     theme = lib.mkOption {