Add progress to *Loading database...* *Decrypting database content...*

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

As far as I understand the database opening takes time according to a given number of cryptographic operations (*Transform rounds* in KeePassXC), however the progress is not shown while waiting:

https://github.com/user-attachments/assets/05af914a-a799-4f68-981d-a1ae300b858a

### Describe the solution you'd like:

I guess that, or there is no cryptographic reason as far as I know not to, the number of cryptographic operations expected to try opening the database is known.

I see 3 use cases showing the progress of these operations:

- paranoid (I guess) people: having set the number of cryptographic operations to take at least multiple seconds, in such case the progress would be appreciated I guess

- regular people: to figure out the progress of the underlying operation, in this case I agree that the very fast/slow loading may be disturbing, contrarily to current regular speed progress (despite cycling), at least it shows that a loading is going on

- abnormal issue: for no apparent reason I already had the decryption prompt for at least tens of seconds (on Fairphone 4), while usually taking about a second, so maybe something else was to blame (rare bug or significant CPU background consumption), but showing progress would have shown if it was stuck or just slow.

### Describe alternatives you've considered:

_No response_

### Additional context:

_No response_

Start by tracing the database-opening and decryption flow described in the issue, including where Transform rounds are handled and where the loading messages are shown. Determine whether progress can be reported reliably, then verify that the loading UI reflects the cryptographic work and remains correct for fast, slow, and unusually delayed openings.

Written by the indexing model from the issue text.