Consolidate coins: a Settings action + a gift-time recommendation

- Settings gains a 'Consolidate coins' card (shown when you hold >1 spendable
  coin): wallet.consolidate() does a max self-send merging them into one, so
  future sends/gifts use a single input — cheaper and a smaller gift QR.
- At gift creation, detect when the gift would bundle several coins
  (giftCoinSummary returns the input count) and trigger the split/consolidate
  offer when count ≥ 3 (not just when it frees change). The offer now explains it
  plainly ('would bundle N coins, costly to claim') and recommends splitting into
  one coin first, with 'Use N coins anyway' as the explicit opt-out.

Verified on regtest: 6 small coins → a 1000-sat gift needs 3 inputs and triggers
the offer; consolidate merges them to 1 coin, after which the same gift is a clean
single input.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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