gui: Refresh empty wallets automatically

[BenWestgate]

Follow-up to #65.

Refresh the eligible Bitcoin Core wallet list once per second while the wallet chooser is visible, matching the CLI's waiting behavior. The selected wallet is preserved by exact name, and polling shares the existing serialized worker boundary so it cannot overlap an import or correction job.

The manual “Check again” action is removed; a wallet created in Bitcoin Core appears automatically. Polling pauses while the chooser is hidden and stops after it leaves the navigation stack.

Validation:

• 974 tests pass normally
• 974 tests pass under python -O
• Ruff check and format pass
• strict mypy passes for src/codex32 and src/codex32_gui
• GTK/Xvfb walkthrough passes, including automatic refresh, exact-name selection preservation, and polling shutdown

[BenWestgate]

AI-generated review, posted at the maintainer's request.

Not ACKing 9b4066e: if the selected wallet disappears on refresh, target falls through to len(current) and silently activates Create a new wallet. Continue can then change the destination without an explicit user choice.

[BenWestgate]

Review note: the polling change does remove the standalone #65 Check again path that dropped restoring, so the stacked #65→#66 candidate fixes that reviewed bug. One separate item still needs maintainer disposition: this PR raises the GUI review budget from 2,000 to 2,050 logical lines. The explicit budget authorization on record is for the installed library (<5200), not for changing the separately enforced GUI cap. Prefer keeping the GUI under 2,000 if that can be done without a risky pre-release refactor; otherwise the 2,050 cap should be an explicit human review/approval rather than an incidental test change.