feat(tts): desktop Kokoro/Piper TTS v2 + user-editable Python path

[anishgoyal1108]

Description

Forward-ports the desktop v2 Kokoro/Piper TTS engine work and adds a
user-editable Python-interpreter override.

• DesktopTTSEngineFactory: lazy createKokoroEngine() (gated on kokoroAvailable)
  • a DesktopKokoroTTSEngineV2 wrapper that delegates synthesis to
    KokoroTTSAdapter, with a per-(utterance, voice, speed) audio cache and bounded
    parallel pre-synthesis (respects the shared TTS-performance setting).
• TTSController: persist + restore the selected EngineType across launches
  (persistedEngineType / saveEngineType).
• Supporting: DesktopTTSService, TTSModule / TTSEngine / TTSState /
  TTSNotificationUseCase, TTSV2ScreenSpec, TTSStateAdapter,
  TTSEngineSettingsScreen, AndroidTTSEngineFactory.
• AppPreferences: kokoroAvailable, selectedTTSEngine, and a new
  kokoroPythonPath override.
• KokoroTTSEngine: honour kokoroPythonPath before auto-discovery, so users whose
  interpreter lives somewhere non-standard (pyenv / conda / venv / system package)
  can point Kokoro at it.

Type of Change

• Core improvement

Testing

• Compiled successfully (desktop) ← reviewer to confirm
• Kokoro synthesis works with auto-discovered Python
• Setting kokoroPythonPath to a specific interpreter is honoured

Additional Notes

Resolved against upstream's TTS refactors: kept upstream's debug log levels while
preserving the new persisted-engine-type hydration. Outstanding: a Settings text
field to edit kokoroPythonPath from the UI (the pref + engine plumbing are done).

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