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>

README.md

@@ -223,6 +223,8 @@ two tables below are split along exactly that line.
 | `nomarchy.fastfetch.enable` | `true` | fastfetch fronted by the themed Nomarchy logo |
 | `nomarchy.keys.enable` | `true` | gpg-agent fronting SSH + pinentry-qt |
 | `nomarchy.displays.enable` | `true` | nwg-displays interactive monitor arranger (helper for `nomarchy.monitors`) |
+| `nomarchy.viewers.enable` | `true` | Document/image viewers: zathura (Stylix-themed PDF) + imv |
+| `nomarchy.mime.enable` | `true` | Default "open with" associations (PDF/image/video/text/browser/directory); entries for apps you removed are skipped, so it degrades with the suite |
 | `nomarchy.monitors` | `[]` | Declarative per-output layout → Hyprland `monitor` rules (applied on hotplug); `,preferred,auto,1` wildcard kept as fallback |
 | `nomarchy.themesDir` | Nomarchy's `themes/` | Where per-theme app overrides are probed |
 | `nomarchy.package` | overlay's `nomarchy-theme-sync` | The theme/state tool package, overridable if you fork it |