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)

shell_demo.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""Rendert die NATIVE Shell (ui/shell.py) mit echten Settings-Sektionen.
+
+    TANINUX_UI=native PYTHONPATH=src python shell_demo.py [--shot=PATH]
+"""
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+
+os.environ.setdefault("TANINUX_UI", "native")
+
+import gi  # noqa: E402
+
+gi.require_version("Gtk", "4.0")
+from gi.repository import GLib, Gio  # noqa: E402
+
+from taninux.gui import style, ui  # noqa: E402
+from taninux.gui.pages import SETTINGS_SECTIONS  # noqa: E402
+
+SHOT = next((a for a in sys.argv if a.startswith("--shot=")), None)
+SHOT = SHOT.split("=", 1)[1] if SHOT else None
+
+
+class App(ui.Application):
+    def __init__(self) -> None:
+        super().__init__(application_id="ch.gabrielevarano.Taninux.ShellDemo",
+                         flags=Gio.ApplicationFlags.DEFAULT_FLAGS)
+
+    def do_activate(self) -> None:
+        style.apply_persisted()
+        win = ui.MainWindow(self, SETTINGS_SECTIONS, "System Settings")
+        win.present()
+        if SHOT:
+            win.fullscreen()
+
+            def _capture() -> bool:
+                subprocess.run(["grim", SHOT], check=False)
+                self.quit()
+                return False
+
+            GLib.timeout_add(1000, _capture)
+
+
+if __name__ == "__main__":
+    raise SystemExit(App().run([]))