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>

tests/CMakeLists.txt

@@ -16,6 +16,7 @@ sloposk_add_test(tst_textrules)
 sloposk_add_test(tst_deadkeys)
 sloposk_add_test(tst_keyrepeater)
 sloposk_add_test(tst_visibility)
+sloposk_add_test(tst_diagnostics)
 
 # Needs the QML module itself, not just the C++ backing library.
 sloposk_add_test(tst_keyboardsurface)