fix: recover gracefully when model files are deleted outside Handy

[DhruvN102]

Fixes #1893

Problem

If a model's files are removed from disk outside Handy, Handy kept checking its cached state instead of checking reality, therefore, Delete and Rescan couldn't clear the stale entry, so there was no way out of the UI short of touching app data directly.

Changes

• Recover gracefully when model files are deleted outside Handy — model
  listing (and Rescan) now verifies a model's files actually exist on disk
  instead of trusting the cached is_downloaded state. A stale entry pointing
  at a missing path moves back to "Available to Download" instead of staying
  listed as present.
• Emit loading_failed for get_model_path errors too — the
  loading_failed event emitter is now defined before the model path
  lookup instead of after, so a failure in get_model_path (e.g. the file no
  longer exists) also emits a terminal loading_failed event instead of
  leaving the frontend stuck on "Loading …" with no feedback.

[cjpais]

Thanks I'll take a look soon!

[DhruvN102]

Looks good, Thanks!