fix: avoid stuck canceling when cache database cannot be opened

[lazydao]

Summary

• catch uncaught exceptions in preparation worker (including cache open failures)
• always reset on_working in finally so cancel flow can complete
• close the dialog path on preparation failure instead of leaving UI stuck in "Canceling..."

Repro

Trigger sqlite3.OperationalError: unable to open database file during advanced translation preparation (for example invalid or unwritable cache path).

Expected

Preparation shows failure details and exits cleanly instead of hanging in canceling state.

Testing

• not run (logic change only)