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/BACKLOG.md

@@ -26,19 +26,6 @@ next, in what order*.
 
 ## NEXT
 
-### 8. Complete-workstation viewers + default applications
-New. The template ships mpv but **no PDF viewer and no image viewer**,
-and nothing sets xdg-mime defaults — so "open a PDF/photo" falls to
-whatever GTK guesses (GIMP for images). Two halves: (a) add a themed,
-lightweight PDF viewer + image viewer to the template's active
-`home.packages` (candidates: zathura or GNOME Papers; imv or loupe —
-prefer what Stylix themes well); (b) a mime-defaults module setting
-`xdg.mimeApps` associations (browser/PDF/image/video/text, `mkDefault`)
-— real config, so unlike bare packages this *is* module territory, but
-it must degrade gracefully when the user deletes a package from the
-suite. **Verify:** V1 + assert the rendered mimeapps.list; V3 queue an
-open-a-file smoke.
-
 ### 9. Update & rollback UX
 New; extends the update-awareness story to the other half: what changed,
 and how do I get back. (a) `sys-update` prints a human diff of what the
@@ -209,7 +196,9 @@ decide.
 - **zram swap:** faster under pressure and pairs with NOW#3, but it
   interacts with the hibernation-swapfile story (resume device/priority
   ordering) — adopt, adopt-with-hibernation-guard, or skip?
-- **Default browser:** the template comments Firefox out; item #8's mime
-  defaults need *something* to point `text/html` at — ship a browser
-  active in the suite, or leave browserless-by-default and let mime
-  defaults degrade?
+- **Default browser:** the template comments Firefox out. The shipped
+  mime defaults (item 8, done) point `text/html`/http(s) at
+  `firefox.desktop` as *inert* entries — they activate the moment
+  Firefox is installed and are skipped otherwise, so the remaining call
+  is only: ship a browser active in the suite, or stay
+  browserless-by-default?