feat(mc): open files with their configured app via xdg-open

Ship `mc.ext.ini` as the `local.mc.extIni` option (like `local.mc.menu`):
Enter browses archives in-place as before and delegates every other type to
`xdg-open`. Add public `mime.nix` with `xdg.mimeApps` defaults (pdf ->
zathura, images -> imv, audio/video -> mpv, office -> libreoffice) -- public
because those handlers are; personal browser/mail defaults stay private.

Also document, at the Hyprland swallow config, how swallowing differs from an
app opening fullscreen/maximized (LibreOffice restores its own maximized state
and looks swallowed but isn't).

modules/home.nix

@@ -81,7 +81,7 @@ let
   '';
 in
 {
-  imports = [ ./gpg.nix ./media.nix ./texlive.nix ./librewolf.nix ./mc.nix ];
+  imports = [ ./gpg.nix ./media.nix ./texlive.nix ./librewolf.nix ./mc.nix ./mime.nix ];
 
   # Put ~/.local/bin (the XDG user-bin dir) on PATH so hand-dropped scripts run
   # by name without wiring each through nix.
@@ -521,7 +521,25 @@ in
         # Matches on the *parent* terminal's class. Needs standalone foot: in
         # server/client mode all windows share one process, so PID ancestry can't
         # tell them apart and swallowing breaks — that's why $mod+Return runs
-        # `foot`, not `footclient`.
+        # `foot`, not `footclient`. Swallow is decided by getSwallower() walking
+        # the new window's parent-PID chain (up to 25 hops) for a window whose
+        # class matches swallow_regex; swallow_exception_regex, if set, matches the
+        # *terminal's title*, not the new window's class (a common misreading).
+        #
+        # Do NOT confuse swallowing with an app opening fullscreen/maximized — they
+        # look identical (the launching terminal vanishes) but are different things:
+        #   - real swallow removes the terminal from the layout and restores it only
+        #     when the swallower closes; you cannot un-swallow a live window.
+        #   - a fullscreen/maximized app merely covers the workspace; the launcher
+        #     is still a tile underneath and $mod+f / $mod SHIFT f toggles it back.
+        # `hyprctl clients` is decisive: a real swallow shows the terminal
+        # hidden=true and the new window swallowing!=0x0. LibreOffice trips this —
+        # it restores its own maximized state on open (from mc or a shell), so a
+        # document appears to "swallow" mc when it is only fullscreen. And being
+        # single-instance (one soffice.bin owns every window via IPC handoff), when
+        # it genuinely does swallow, that is fixed by whatever launched its daemon,
+        # not per-open — so it can't do the tile-from-mc / swallow-from-shell split
+        # that single-process handlers like zathura/imv get for free.
         enable_swallow = true;
         swallow_regex = "^(foot)$";
       };