Improve developer experience and navigation

[sri-rang]

Summary

• consolidate the main UI into vertical Offline Dictation, Speech Models, and Settings views
• add editable, failure-aware KDE global shortcut handling with live synchronization
• make default builds and runs incremental while preserving explicit validation targets
• keep user-downloaded models during uninstall and strengthen installation smoke coverage
• require rebase merges in the repository PR workflow

Validation

• make lint — passed
• make test — 7/7 tests passed
• make coverage BUILD_DIR=build-coverage BUILD_TYPE=Debug CMAKE_ARGS=-DKASTWORD_FETCH_DEFAULT_MODEL=OFF — passed (80.5% lines, 69.9% branches)
• ASAN_OPTIONS=detect_leaks=1:halt_on_error=1 UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 make test BUILD_DIR=build-sanitizers BUILD_TYPE=Debug CMAKE_ARGS="-DKASTWORD_ENABLE_SANITIZERS=ON -DKASTWORD_FETCH_DEFAULT_MODEL=OFF" — 7/7 tests passed
• make install-smoke — passed
• make validate — passed