Stop telling every desktop that it is KDE

Only GNOME was recognised, and everything else was handed to KWin. On i3,
XFCE, Cinnamon, MATE, sway and the rest, Dikte wrote an entry into
kglobalshortcutsrc that nothing reads, called the session KDE, and
promised that the keys would work after the next login. They never did.

There is no backend to write for any of them. The /dev/input listener is
already desktop-agnostic, so those sessions are the case macOS has always
been: no registry, nothing to install, nothing to remove, and the
combination held by the running process. One backend() function decides
which of the four this session has, and the name shown, the status read
back, what Install writes, what Settings explains and what the installer
promises are all taken from it, so they cannot disagree.

A desktop now only counts when the program that writes its registry is
there too. A GNOME session without gsettings and a Plasma one without
kwriteconfig6 fall to the listener rather than to a file, which is also
how Plasma 5 stops erroring on a kwriteconfig6 it never had.

What Settings shows on those desktops is the truth: no Install button, no
KWin, no listener checkbox (it is the mechanism, not a choice), what
reading /dev/input costs, that the focused application sees the keys too,
and the command to bind if you would rather your desktop owned them. The
evdev listener records what it is listening for the way the Carbon one
does, so the status line has something to say there at all.

Closes #28

settings_ui.py

@@ -1363,15 +1363,34 @@ class SettingsWindow(QDialog):
             )
         elif hotkey.installs_shortcuts():
             explanation = t("The shortcut starts working as soon as it is installed.")
-        else:
+        elif hotkey.backend() == hotkey.MACOS:
             explanation = t(
                 "Dikte asks macOS for these combinations itself, while it is "
                 "running. Nothing is installed, and no other application receives "
                 "them in the meantime."
             )
+        else:
+            # The desktops nobody writes a backend for. Saying "installed" here
+            # would be the old bug in words: there is no registry, the listener
+            # is the whole mechanism, and both of its costs are permanent
+            # rather than lasting until the next login.
+            explanation = t(
+                "{desktop} keeps no shortcut registry, so Dikte listens for "
+                "these combinations itself while it is running. Your user has "
+                "to be able to read /dev/input for that, and the focused "
+                "application receives the keys as well. To have the desktop own "
+                "them instead, bind this command in its own configuration, with "
+                "the last word swapped for pause, cancel, ask or meeting:",
+                desktop=hotkey.desktop_name(),
+            )
         note = QLabel(explanation)
         note.setWordWrap(True)
         layout.addWidget(note)
+        if hotkey.backend() == hotkey.LISTENER:
+            command = QLineEdit(ipc.command_for("toggle"))
+            command.setReadOnly(True)
+            command.setCursorPosition(0)
+            layout.addWidget(command)
         layout.addStretch(1)
         return page
 
@@ -1442,7 +1461,7 @@ class SettingsWindow(QDialog):
         """The field a global shortcut is typed or picked in."""
         box = QComboBox()
         box.setEditable(True)
-        box.addItems(MAC_SHORTCUTS if hotkey.desktop_name() == "macOS"
+        box.addItems(MAC_SHORTCUTS if hotkey.backend() == hotkey.MACOS
                      else SHORTCUTS)
         box.setCurrentText("")
         if placeholder:
@@ -1491,8 +1510,9 @@ class SettingsWindow(QDialog):
     def _install_buttons(install_handler, remove_handler):
         """Install and Remove, where this system has somewhere to install into.
 
-        macOS has not: Dikte asks for the combination itself while it runs, so
-        there is nothing to write down and nothing to take back out.
+        macOS has not, and neither has a Linux desktop that keeps no registry:
+        Dikte holds the combination itself while it runs, so there is nothing
+        to write down and nothing to take back out.
         """
         if not hotkey.installs_shortcuts():
             return []
@@ -1990,11 +2010,16 @@ class SettingsWindow(QDialog):
     def _refresh_shortcut_status(self, which):
         _box, status, missing = self._shortcut_rows[which]
         current = hotkey.shortcut_status(hotkey.SHORTCUTS[which].desktop_id)
-        status.setText(
-            t("Registered in {desktop}: {shortcut}",
-              desktop=hotkey.desktop_name(), shortcut=current) if current
-            else missing
-        )
+        if not current:
+            status.setText(missing)
+        elif hotkey.installs_shortcuts():
+            status.setText(t("Registered in {desktop}: {shortcut}",
+                             desktop=hotkey.desktop_name(), shortcut=current))
+        else:
+            # Nothing was written anywhere: this is the combination the running
+            # process is holding, which is the only sense in which it exists.
+            status.setText(t("Held by Dikte while it runs: {shortcut}",
+                             shortcut=current))
 
     def _cleanup_provider_changed(self):
         provider = self.cleanup_provider.currentData() or "openrouter"