Fix: CAPITALISM tax & transaction error handling bugs

[daniel-skopek]

Balance check fixes

• PurchaseService & ListedItemsButton: CAPITALISM tax now included in
  pre-purchase balance check (was checking base price only, allowing
  buyers who couldn't afford price+tax to pass)
• PurchaseService: item status now restored from IS_BEING_PURCHASED
  to previous status on insufficient funds, preventing items from
  becoming permanently unclickable in the UI

Transaction error handling (ZAuctionManager)

• withdraw() wrapped in try-catch; throws on failure to abort
  the purchase via existing CompletableFuture exception handler
• deposit() to seller wrapped in try-catch with automatic buyer
  refund on failure
• Seller transaction before field corrected for deferred deposits
  (was balance - amount even when no deposit occurred)

Claim service fixes

• Removed fire-and-forget runAsync for deposits; now synchronous
  before marking transactions as RETRIEVED, preventing money loss
  on server crash
• Added try-catch around all deposit calls with error logging