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>

src/backend/KWinInputMethod.cpp

@@ -70,9 +70,11 @@ void KWinInputMethod::setPreedit(const QString &text)
         m_context->preedit(text);
 }
 
+// KWin drops the input method's modifier mask for most clients (it forwards
+// only the key), so Ctrl+A would type "a": combos hold real modifier keys.
 void KWinInputMethod::sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers)
 {
-    if (m_context)
+    if (m_context && (modifiers == Qt::NoModifier || !m_fakeInput.canSendKeysyms()))
         m_context->sendKeysym(keysym, modifiers);
     else
         m_fakeInput.sendKeysym(keysym, modifiers);