gui: native libadwaita-free UI backend + dark shell; packaging/iso updates

- gui/ui: native GTK4 widget vocabulary (.tn-*) behind the ui.* seam,
  default backend now native; style_apple.css design language
- shell: sidebar/topbar/content all #0f0f0f in dark (matches eww panels)
- new pages: wine, online_accounts
- packaging: tanin-icons (accent folder theme), tanin-calendar, gtklock,
  taninux-gtk3-theme, flatpak install handler
- iso/distro: profile + manifest tweaks (ISO binary not included)

packaging/tanin-calendar/ch.gabrielevarano.TaninCalendar.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=Calendar
+Name[de]=Kalender
+Comment=Termine aus deinen Kalendern (Evolution Data Server)
+Comment[de]=Termine aus deinen Kalendern (Evolution Data Server)
+Exec=tanin-calendar
+Icon=x-office-calendar
+Terminal=false
+Categories=Office;Calendar;GTK;
+Keywords=Kalender;Termine;Agenda;Calendar;Events;
+StartupNotify=true
+StartupWMClass=ch.gabrielevarano.TaninCalendar