Wallet migration includes rescan #930

Users may want to migrate legacy wallets they haven't used in years. The migration itself is typically very quick, but it's followed by a rescan (if the wallet is behind).

Depending on whether the user has -blockfilterindex enabled, this rescan takes somewhere between long and really really long.

There's no visual indication, only the logs show what's going on:

<img width="383" alt="Image" src="https://github.com/user-attachments/assets/da911b44-8052-4b90-aa16-67625e550e3e" />

Ideally the migration should finish and then prompt the user that a rescan is about to happen. That way the user can abort the rescan and resume it some other time if it takes too long.

Alternatively, the migrate wallet dialog could at least warn that a rescan is (or may be) happening and could take a while.

Fun aside: block filters don't work on legacy wallets, so it's much faster to first migrate and then rescan.

Moved from: https://github.com/bitcoin/bitcoin/issues/32045.

This is not a GUI issue.

Pease elaborate. It's asking for visual output, in the GUI.

Easy

Opened a fix in #943.

The problem is that MigrateWalletActivity::do_migrate() calls migrateWallet() as a single blocking operation that performs both the wallet conversion and the blockchain rescan internally. During the entire wait, the progress dialog shows "Migrating Wallet…" with no indication that a potentially hours-long rescan is running, the only way to know is to tail the debug log.

Taking the alternative approach mentioned in the issue since separating migration from rescan would require wallet backend changes. The fix adds a rescan warning to the informative text of both confirmation dialogs (migrate() and restore_and_migrate()) so users know what to expect before clicking Confirm, and updates the progress dialog label in do_migrate() to state that a blockchain rescan will follow, giving context during the wait.