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>

README.md

@@ -26,14 +26,17 @@ Needs Qt ≥ 6.8 (Quick, WaylandClient, DBus, Test), KF6 WindowSystem, KF6 Confi
 
 ## Run
 
-KWin only offers the input-method protocol to the process it launches itself.
+KWin offers the input-method protocol and fake input (held keys, shortcuts, typing outside text fields) to the keyboard it launches itself, so launch sloposk through KWin.
 
 | Where | How |
 |---|---|
 | Nested session (safe) | `kwin_wayland --no-lockscreen --socket sloposk-dev --width 1400 --height 900 --inputmethod $PWD/build/src/sloposk kwrite` |
 | Desktop | `cmake --install build --prefix ~/.local`, then System Settings → Keyboard → Virtual Keyboard → **sloposk** |
+| Desktop, from the build | `ln -s $PWD/build/src/org.sloposk.keyboard.desktop ~/.local/share/applications/`, then pick **sloposk** as above |
 | UI only | `build/src/sloposk` (stays visible, types nowhere) |
 
+Started any other way (e.g. `kioclient exec`), KWin grants neither protocol; the ⚙ panel lists what's missing.
+
 Show/hide, e.g. from a global shortcut: `qdbus org.sloposk.Keyboard /Keyboard org.sloposk.Keyboard.Toggle`
 
 Dev hooks: `SLOPOSK_AUTOTYPE=text` commits on focus; `SLOPOSK_SCREENSHOT=file.png` saves the window and quits.