Combine DatabaseWidget load/unlock handling

[MichaeINeumann]

Summary

• Route DatabaseOpenWidget completion to DatabaseWidget::unlockDatabase.
• Merge open-widget and dialog unlock paths while preserving intent handling and open-widget-only behavior (expired-entry search, close-on-cancel, save attempt reset).
• Remove the redundant loadDatabase slot.

Testing

• Set "On database unlock, show entries that will expire within" to 0 days.
• Created DB A with an expired entry and DB B as a second database.
• Opened DB A via the embedded open widget and verified the "Expired entries" search.
• Locked DB A, selected a non-root group/entry, unlocked in the open widget, verified focus restore and no expired search on re-unlock.
• Canceled the open widget while locked and confirmed the tab closed.
• Started "Merge from Database" with DB B, unlocked via dialog, confirmed merge dialog appears and cancel shows the "Merge canceled" message.

Fixes #3763

AI

• AI-assisted: OpenAI GPT-5 (Codex)

[droidmonkey]

This looks great thank you