Wallet: Rescan imported mnemonic wallets

[Tabakkui]

Problem

When a first-run wallet is created from an imported BIP39 mnemonic while the chain already has history, CWallet::CreateWalletFromFile writes the current chain locator before its startup rescan. The subsequent rescan therefore starts at the tip and restored funds are not discovered until a manual rescan. This is the behavior reported by #1081.

Approach

• Track whether the first-run wallet uses an imported mnemonic, including the GUI restore path.
• Do not persist the current best-chain locator before the imported-mnemonic rescan.
• Scan from the genesis block without applying the new-wallet birthday shortcut, then persist the locator after the scan.
• Preserve the existing fast path for newly generated wallets.

Test plan

• git diff --check
• python3 -m py_compile test/functional/wallet_mnemonic_rescan.py
• Structural verification: 7/7 checks passed, including comparison against the pre-fix origin/develop source shape.
• Added test/functional/wallet_mnemonic_rescan.py, which funds a deterministic address, removes wallet.dat, recreates the wallet from the same mnemonic, and checks the restored balance.
• The functional test was attempted before and after the fix but cannot start because this ARM64 checkout has no built ravend binary or configured Makefile (FileNotFoundError: [Errno 2] No such file or directory: "ravend").

Risk and exclusions

This changes only first-run imported-mnemonic wallet initialization and its rescan anchor. Newly generated wallets retain the existing best-chain shortcut. It does not change consensus rules, mnemonic derivation, or the general rescan scheduler.

Fixes #1081