refactor(tui): simplify TUI with guard-clause helper and cleanup

Extract _get_current_param helper to eliminate repeated guard-clause
boilerplate in _apply_* and action_toggle_* methods. Normalize
_apply_media_theme to use _run_command instead of a custom inline
worker. Consolidate _ButtonVariant type alias into widgets.py as
ButtonVariant. Remove unused FireplaceSelector class. Move frequently
used deferred imports (FlameEffectParam, HeatParam, LightStatus,
TempUnitParam, TimerParam, TimerStatus) to module top level.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: deviantintegral