FIX: move AI model storage out of ~/Documents to Application Support

[osterowsky]

What

SpeakType was silently creating a huggingface/ folder inside the user's ~/Documents on every launch, even before any model was downloaded. This is a
violation of Apple's file system guidelines — ~/Documents is the user's personal space, not a place for app data.

Changes

• No more eager folder creation on launch — the storage directory is only created when a model download actually begins
• Moved default storage from ~/Documents/huggingface/ to ~/Library/Application Support/SpeakType/Models/ (Apple's prescribed location for persistent
  app data)
• Fixed tokenizer download path — WhisperKit downloads tokenizer configs (openai/whisper-*) separately from the CoreML models; both are now routed to
  the same Application Support location via tokenizerFolder in WhisperKitConfig
• Fixed a pre-existing crash in DashboardView where .isEmpty was called on a tuple (day: String, count: Int)

Why it matters

Users who uninstalled SpeakType still found a huggingface/ folder sitting in their Documents. With this fix, the app leaves no trace in user-facing directories unless a model has been explicitly downloaded.