Balances: add detailed available, pending, immature, total, and watch-only display

[johnny9]

Code Gap

The QML wallet shell currently exposes a single selected-wallet balance from WalletQmlModel::balance(), backed by interfaces::Wallet::getBalance(). Qt's overview shows separate spendable, pending, immature, and total balances, and it can expose watch-only balance sections.

The current single-balance badge is an initial implementation. The detailed breakdown is a secondary enhancement to make balance state clearer before v1.0.

Work Needed

• Expose release-scoped balance fields from the wallet model: available/spendable, pending, immature, total, and watch-only where applicable.
• Add a QML balance overview surface or expanded wallet badge/details surface.
• Hide zero/irrelevant rows consistently with the selected release behavior.
• Refresh all balance fields on wallet transaction/status changes and display-unit changes.
• Add coverage for confirmed, unconfirmed, immature coinbase, and watch-only balances.

Priority

v1.0 / P2 secondary balance enhancement.