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)

page_demo.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python3
+"""Lädt eine ECHTE TANINUX-Seite über die ui-Naht im native-Backend.
+
+    TANINUX_UI=native PYTHONPATH=src python page_demo.py defaultapps [--dark] [--shot=PATH]
+
+Beweist die API-Gleichheit: derselbe Seiten-Code (gui/pages/<name>.py) rendert
+unverändert sowohl mit Adw als auch mit den nativen Apple-Optik-Widgets — je
+nach TANINUX_UI. Hier erzwingen wir native.
+"""
+from __future__ import annotations
+
+import importlib
+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, Gtk  # noqa: E402
+
+from taninux.gui import ui  # noqa: E402
+
+PAGE = next((a for a in sys.argv[1:] if not a.startswith("-")), "defaultapps")
+DARK = "--dark" in sys.argv
+SHOT = next((a for a in sys.argv if a.startswith("--shot=")), None)
+SHOT = SHOT.split("=", 1)[1] if SHOT else None
+
+
+def on_activate(app: Gtk.Application) -> None:
+    ui.ensure_css()
+    mod = importlib.import_module(f"taninux.gui.pages.{PAGE}")
+
+    win = Gtk.ApplicationWindow(application=app, title=f"{PAGE} · native")
+    win.set_default_size(720, 760)
+    ui.set_dark(win, DARK)
+    win.set_titlebar(Gtk.HeaderBar())
+    win.set_child(mod.build())
+    win.present()
+
+    if SHOT:
+        win.fullscreen()
+
+        def _capture() -> bool:
+            subprocess.run(["grim", SHOT], check=False)
+            app.quit()
+            return False
+
+        GLib.timeout_add(900, _capture)
+
+
+def main() -> int:
+    app = Gtk.Application(application_id="ch.gabrielevarano.Taninux.PageDemo")
+    app.connect("activate", on_activate)
+    return app.run([])
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())