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/Main.qml

@@ -7,6 +7,7 @@ Window {
     required property LayoutModel layoutModel
     required property KeyboardController controller
     required property Settings settings
+    required property Diagnostics diagnostics
     property var placement: null // FloatingWindow, set from C++
     property bool settingsOpen: false
     // Height of the key area, for C++ keeping key sizes when rows change.
@@ -58,6 +59,7 @@ Window {
             visible: root.settingsOpen
             settings: root.settings
             layoutModel: root.layoutModel
+            diagnostics: root.diagnostics
         }
 
         DragHandle {
@@ -69,6 +71,7 @@ Window {
         SettingsButton {
             anchors { left: parent.left; leftMargin: 8; bottom: parent.bottom }
             checked: root.settingsOpen
+            alert: root.diagnostics.problems.length > 0
             onClicked: root.settingsOpen = !root.settingsOpen
         }