Floating polish: visibility, resize, saved geometry, D-Bus, desktop entry

- VisibilityController: show on field focus, dismiss until next focus,
  manual show/hide/toggle overrides
- FloatingWindow: resize from the corner grip, clamp to screen
- WindowSettings: geometry per screen, debounced saves
- DBusControl: org.sloposk.Keyboard Show/Hide/Toggle for global shortcuts
- org.sloposk.keyboard.desktop so System Settings lists it as a virtual keyboard
- README with build/run/layout docs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

data/org.sloposk.keyboard.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Name=sloposk
+Comment=iPad-style floating on-screen keyboard
+Exec=sloposk
+Type=Application
+Icon=input-keyboard-virtual
+NoDisplay=true
+X-KDE-Wayland-VirtualKeyboard=true