Report unavailable protocols and services; send combos as real keys

Diagnostics collects what this session doesn't allow, logs it, and lists
it in the settings panel (with a dot on the gear): KWin not launching
sloposk as its keyboard (detected by WAYLAND_SOCKET, since KWin offers the
input-method global to every client), fake input missing or too old,
org.kde.keyboard unreachable, and the D-Bus control name taken.

Modifier combos go through fake input even in text fields: KWin drops
the input method's modifier mask for most clients, so Ctrl+A typed "a".

The desktop file is generated with an absolute Exec, which KWin needs to
match it: one for the installed binary, one for the build's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

qml/SettingsButton.qml

@@ -5,6 +5,8 @@ Item {
     id: button
 
     property bool checked: false
+    // Something doesn't work (see Diagnostics): a dot invites a look in settings.
+    property bool alert: false
     signal clicked()
 
     implicitWidth: 36
@@ -17,6 +19,16 @@ Item {
         color: tap.pressed ? Theme.text : Theme.handle
     }
 
+    Rectangle {
+        visible: button.alert && !button.checked
+        width: 6
+        height: 6
+        radius: 3
+        color: Theme.warning
+        x: parent.width / 2 + 6
+        y: 3
+    }
+
     TapHandler {
         id: tap
         onTapped: button.clicked()