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/ProblemList.qml

@@ -0,0 +1,26 @@
+import QtQuick
+
+// What doesn't work in this session and why (see Diagnostics); hidden when all is well.
+Column {
+    id: list
+
+    property list<string> problems
+
+    visible: problems.length > 0
+    spacing: 4
+
+    SectionHeader { text: qsTr("Not Available"); color: Theme.warning }
+
+    Repeater {
+        model: list.problems
+
+        Text {
+            required property string modelData
+            width: list.width
+            text: "⚠ " + modelData
+            color: Theme.text
+            font.pixelSize: 13
+            wrapMode: Text.Wrap
+        }
+    }
+}