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/tanin-libadwaita.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=TANINUX libadwaita theme follower (Fuji light/dark on color-scheme change)
+PartOf=graphical-session.target
+After=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/tanin-libadwaita watch
+Restart=on-failure
+RestartSec=2
+
+[Install]
+WantedBy=graphical-session.target