test: extend launch smoke coverage for secondary windows

[TimInTech]

Summary

• Adds test_secondary_windows_instantiate to tests/test_smoke_launch.py
• Smoke-tests three previously uncovered dialog/panel constructors under QT_QPA_PLATFORM=offscreen WHISPER_GUI_TESTS=1:
  • HistoryPanel via app.show_history_panel() (lazy-init path, same as production)
  • TtsWindow via app.show_tts_window() (lazy-init path)
  • SettingsDialog(config) directly — show_settings_dialog() calls exec() which would block; direct instantiation is the correct offscreen strategy
• Completes the smoke-coverage goal: MainWindow, Tray, Compose, Settings, TTS, and History are now all exercised

No runtime side effects

• Config isolated via tmp_path fixture; real user config never read
• stop_hotkey_worker() called before any window is shown; no real evdev thread
• No audio, no OpenAI/Whisper/Piper calls, no xclip/paste
• No XDG_RUNTIME_DIR manipulation
• try/finally resets language state to prevent bleed into subsequent tests

Test plan

• QT_QPA_PLATFORM=offscreen WHISPER_GUI_TESTS=1 python -m pytest tests/test_smoke_launch.py -v → 4 passed
• Full suite python -m pytest -q → 395 passed
• python3 -m compileall app tests → OK
• git diff --check → no whitespace errors
• Secret scan → clean (no keys, tokens, credentials)
• Code-reviewer agent → APPROVED, no blocking issues