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/mime.nix

@@ -0,0 +1,48 @@
+# Default applications (xdg mimeapps.list) — without this, "open a
+# PDF/photo" falls to whatever GTK guesses first (GIMP for images).
+# Every association is mkDefault AND degrades gracefully by design: an
+# entry whose .desktop file isn't installed is skipped by GIO/xdg-open,
+# which then falls through to whatever else claims the type — so
+# deleting an app from the template suite (or never uncommenting the
+# browser) leaves no broken "open" behaviour, just the old guessing.
+{ config, lib, ... }:
+
+lib.mkIf config.nomarchy.mime.enable {
+  xdg.mimeApps = {
+    enable = lib.mkDefault true;
+    defaultApplications = lib.mapAttrs (_: v: lib.mkDefault v) {
+      "application/pdf" = "org.pwmt.zathura.desktop";
+
+      "image/png" = "imv.desktop";
+      "image/jpeg" = "imv.desktop";
+      "image/gif" = "imv.desktop";
+      "image/webp" = "imv.desktop";
+      "image/avif" = "imv.desktop";
+      "image/bmp" = "imv.desktop";
+      "image/tiff" = "imv.desktop";
+      "image/svg+xml" = "imv.desktop";
+
+      "video/mp4" = "mpv.desktop";
+      "video/webm" = "mpv.desktop";
+      "video/x-matroska" = "mpv.desktop";
+      "video/quicktime" = "mpv.desktop";
+      "audio/mpeg" = "mpv.desktop";
+      "audio/flac" = "mpv.desktop";
+      "audio/ogg" = "mpv.desktop";
+
+      # The template's active editor; degrades if you drop vscode.
+      "text/plain" = "code.desktop";
+
+      # The system-side Thunar (nomarchy.system.fileManager).
+      "inode/directory" = "thunar.desktop";
+
+      # The template ships no browser by default (open Decision) — these
+      # are inert until one is installed; firefox is the template's first
+      # suggestion. A different browser registers its own handler and
+      # wins once these entries stay dead.
+      "text/html" = "firefox.desktop";
+      "x-scheme-handler/http" = "firefox.desktop";
+      "x-scheme-handler/https" = "firefox.desktop";
+    };
+  };
+}