fix: Qt wallet mint fails on fragmented UTXOs — missing auto-consolidation

[JohnnyLawDGB]

Summary

• Bug: Qt GUI mints fail with "Too many small UTXOs" when the wallet has many coinbase-sized UTXOs (~1,078 DGB each), because BuildMintTransaction enforces a 400-input limit (~431,400 DGB max) and the Qt path had no consolidation fallback
• Root cause: PR fix: auto-consolidate fragmented UTXOs when minting DigiDollar #391 added UTXO auto-consolidation to the RPC mintdigidollar handler (src/rpc/digidollar.cpp:983-1111) but the Qt wallet's mintDigiDollar (src/qt/walletmodel.cpp) was never updated with the same logic — it called BuildMintTransaction once and returned the error directly on failure
• Fix: Port the identical multi-pass consolidation logic (1400 inputs/pass, 10 max passes) into the Qt mint path, so GUI users get the same transparent consolidation-then-retry behavior as RPC users

Reproduction (reported by Aussie and DanGB on RC29 testnet)

Mint Amount	Collateral Needed	400-Input Capacity	Result
$100 DD (tier 0)	~231,000 DGB	~431,400 DGB	Pass
$200 DD (tier 0)	~462,000 DGB	~431,400 DGB	Fail
$300 DD (tier 0)	~694,000 DGB	~431,400 DGB	Fail

DanGB has 11M tDGB across thousands of ~1,078 DGB coinbase UTXOs. Aussie has a similar distribution. Both confirmed $100 works but $200+ fails in Qt. The RPC path would have auto-consolidated and succeeded.

How it works

1. BuildMintTransaction attempts the mint — if it fails with "Too many small UTXOs":
2. Check total balance covers collateral requirement
3. Multi-pass loop: sweep up to 1400 UTXOs per pass into a single consolidation output (wallet change address), broadcast each consolidation tx, refresh the UTXO set
4. Single-pass fallback: if total UTXOs <= 1400, one consolidation tx suffices
5. Retry BuildMintTransaction with the consolidated UTXO set

Test plan

• Qt wallet mint with >400 small UTXOs now auto-consolidates and succeeds
• Qt wallet mint with <400 UTXOs still works without consolidation (no regression)
• RPC mintdigidollar behavior unchanged
• Consolidation tx appears in wallet history
• Error message shown if total balance truly insufficient after consolidation attempt

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