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>

agent/JOURNAL.md

@@ -17,6 +17,26 @@ Template:
 
 ---
 
+## 2026-07-04 — viewers + mime defaults (iteration #17, item 8)
+- **Task:** BACKLOG NEXT#8 — PDF/image viewers + xdg-mime defaults.
+- **Did:** modules/home/viewers.nix (`nomarchy.viewers.enable`, default
+  on): programs.zathura (Stylix themes it — target added to stylix.nix's
+  explicit list, autoEnable=false bit us first try) + imv.
+  modules/home/mime.nix (`nomarchy.mime.enable`): mkDefault
+  xdg.mimeApps for pdf→zathura, image/*→imv, av→mpv, text→code,
+  dir→thunar, html/http(s)→firefox as INERT entries (browser Decision
+  untouched — absent .desktop entries are skipped by GIO). README rows
+  added (option-docs enforces them now). DEVIATION from the item text:
+  viewers are a module toggle, not template packages — zathura's
+  theming needs its HM module, which is exactly the "real config"
+  boundary of the toggle discipline.
+- **Verified:** V0; V1 — template-home generation builds; asserted
+  rendered mimeapps.list (all associations), themed zathurarc
+  (Tokyo Night rgb), imv/zathura in home-path. V3 smoke queued.
+- **Pending:** V3 open-a-file smoke; browser Decision now smaller
+  (inert entries ship; only "active in suite?" remains).
+- **Next suggestion:** NEXT#9 (update & rollback UX) — or #10 doctor.
+
 ## 2026-07-04 — docs slice (c): install story — item 6 COMPLETE (iteration #16)
 - **Task:** BACKLOG NOW#6 slice (c), the last one — install/first-run
   narrative end to end.