Add ParticipantShare type and client-side rebalance/validation helpers for purchase splits (rebalancePurchaseSplit, recalculatePercentages, calculateRemainingToAllocate, validatePurchaseDraft). Integrate rebalancing and validation into the miniapp ledger UI (auto-adjust shares, percentage<>amount syncing, remaining/error display, prefill participants on new purchase, disable save when invalid). On the backend, tighten input validation in finance-command-service for custom_amounts (require explicit shares and sum match) and make settlement-engine more lenient when reading legacy/malformed custom splits (ignore missing shares, accept explicit subset). Also add a test ensuring dashboard generation doesn't 500 on legacy malformed purchases.
