Wire niri desktop: tanin-eww, tanin-desktop (niri), tanin-setup theme follower

- tanin-eww: package eww shell config + niri config -> /usr/share/tanin/skel/.config
  (dev paths rewritten to ~/.config/eww; niri spawn-at-startup uses sh -c)
- tanin-desktop: switch metapackage from Hyprland to the niri stack
  (niri, greetd+tanin-greet, cage, eww stack, xdg-desktop-portal-gnome, fonts)
- tanin-setup: apply Fuji theme + enable tanin-libadwaita.service on first run
- tanin-libadwaita: ship systemd user service (color-scheme watcher)
- distro manifest synced; drop stale regreet references

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/tanin-libadwaita/PKGBUILD

@@ -21,4 +21,6 @@ package() {
   install -Dm644 "$startdir/gtk-3.0/dark.css"  "$pkgdir/usr/share/tanin/libadwaita/gtk-3.0/dark.css"
   install -Dm644 "$startdir/gtk-3.0/light.css" "$pkgdir/usr/share/tanin/libadwaita/gtk-3.0/light.css"
   install -Dm755 "$startdir/tanin-libadwaita"  "$pkgdir/usr/bin/tanin-libadwaita"
+  install -Dm644 "$startdir/tanin-libadwaita.service" \
+    "$pkgdir/usr/lib/systemd/user/tanin-libadwaita.service"
 }