GUI: Add a use-available-balance action

[Tabakkui]

Problem

Users must manually copy their available balance into the send amount field.

Approach

• Add a Use available balance button to each normal send entry.
• Use the wallet model balance with the existing coin-control selection.
• Subtract amounts already entered in other visible send entries.
• Enable fee subtraction so the full spendable balance remains the transaction total.
• Add Qt wallet-test coverage for the button, amount population, and fee checkbox.

Test plan

• git diff --check — passed.
• XML parse of src/qt/forms/sendcoinsentry.ui — passed.
• Qt test binary/build is not available in the checkout; the Pi was not used for a full Ravencoin build.

Risk and exclusions

Focused GUI/wallet UX change only. No consensus, RPC, network, wallet serialization, or translation-only changes.

Fixes #1269