Fix Piper TTS voices stuck on "Loading voices..."

[Copilot]

Switching to Piper TTS left the Voice tab permanently showing "Loading voices…" — voices never populated and the dropdown remained unusable.

Two independent bugs caused this:

• parent_window never wired up (settings_window.py) — VoiceTab was constructed without parent_window, so _load_voices() fetched voices in the background thread but the if self.parent_window: guard silently dropped the after() callback, meaning _apply_voices_ui() never ran.

• Provider-unaware voice cache (tts_engine.py) — get_available_voices() used a purely time-based cache. After switching providers the still-fresh cache returned the previous provider's voices; the Voice tab would silently display stale Edge TTS voices while Piper was active (or vice versa).

Changes

• src/gui/settings_window.py — pass parent_window=self.window when constructing VoiceTab.

• src/tts/tts_engine.py

  • Add _cached_provider: str field alongside the existing time-based cache.
  • Cache is only reused when both the timestamp and provider match; any provider change triggers a fresh fetch.
  • clear_voices_cache() now also resets _cached_provider.
  • Extract _get_active_provider_name() helper to avoid duplicating the settings lookup.

# Before: cache checked time only
if self._voices_cache is not None and time.time() - self._cache_timestamp < self._cache_duration:
    return self._voices_cache

# After: cache is also invalidated when the active provider changes
active_provider = self._get_active_provider_name()
if (
    self._voices_cache is not None
    and self._cached_provider == active_provider
    and time.time() - self._cache_timestamp < self._cache_duration
):
    return self._voices_cache

• tests/test_tts_engine_cache.py — four new unit tests covering: same-provider cache hit, cache bust on Edge→Piper switch, cache bust on Piper→Edge switch, and clear_voices_cache() resetting provider tracking.

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