feat: file manager — yazi (flagship TUI) + Thunar (GUI)

Legacy shipped only Thunar (and a stray nautilus extension, mid-migration).
This pairs a keyboard-driven TUI flagship that fits the distro's identity
with the GUI half for point-and-click + the "open folder" handler.

yazi (modules/home/yazi.nix, nomarchy.yazi.enable, SUPER+E, `y` shell
wrapper cd's on exit): themed from theme-state.json (mgr/mode/status
mapped from the palette; merges over yazi's defaults so it tracks the
theme) with a curated, correctly-wired plugin set —
  full-border, git (setup + prepend_fetchers), smart-enter (l/Enter),
  chmod (cm), mount (M), compress (caa), mediainfo (audio/subtitle only —
  images/videos keep native thumbnails), glow (markdown previews).
Preview/tooling deps: ffmpegthumbnailer, poppler-utils, p7zip, jq, fd,
ripgrep, fzf, chafa, mediainfo, glow.

Thunar (modules/nixos/file-manager.nix, nomarchy.system.fileManager.enable):
GTK file manager (Stylix-themed) + archive/volman/media-tags plugins +
gvfs/tumbler/udisks2 backends + thumbnailers + exo. udisks2 also backs
yazi's mount plugin.

Verified: flake check green, HM generation builds (yazi TOML/init.lua
render, all 8 plugins resolve), every package present in the live ISO
closure (offline install draws from it — no source-build leak).
Not verified: yazi's runtime preview/plugin behaviour needs a real
session (TUI; can't drive in CI here).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/nixos/default.nix

@@ -11,7 +11,7 @@ let
   cfg = config.nomarchy.system;
 in
 {
-  imports = [ ./options.nix ./plymouth.nix ];
+  imports = [ ./options.nix ./plymouth.nix ./file-manager.nix ];
 
   config = {
     # The safe half of distro branding: distroName flows into