Add explicit connection modes

[daredoole]

Summary

• add connection_mode=powertoys|inputflow|hybrid to config, CLI, doctor, GUI, and Zenity controller
• gate the PowerToys transport so inputflow mode can run without Windows host/key
• keep Android/InputFlow peers available only in inputflow or hybrid modes

Validation

• bash -n mwb-desktop-ui.sh
• scripts/validate-rpm-packaging.sh
• cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
• cmake --build build --parallel 2
• ctest --test-dir build --output-on-failure
• timeout 2 ./build/mwb_client run --connection-mode inputflow --disable-clipboard
• cmake -S . -B build-ci-min -DCMAKE_BUILD_TYPE=Release -DPKG_CONFIG_EXECUTABLE=/bin/false
• cmake --build build-ci-min --parallel 2
• ctest --test-dir build-ci-min --output-on-failure