Unify transaction toast notifications (issue #5)

[DingiDinigi]

Problem

Transaction toasts (PENDING, SUCCESS, ERROR) were implemented inconsistently across the codebase — some success toasts linked to the block explorer, others didn't; error toasts had inconsistent dismiss behavior; and at least one component (SwapCard.tsx) bypassed the shared toast system entirely with raw sonner calls.

Changes

• components/TxToasts.tsx
  • Fixed a bug where error toasts (notifyTxError and the error branch of notifyTx) were missing duration: Infinity, causing them to auto-dismiss instead of requiring manual dismissal as required by the acceptance criteria.
  • Added notifyTxSuccessLocal(message) — a success toast with the same unified styling/duration as notifyTxSuccess, for actions that succeed without producing a real on-chain transaction hash (so we don't fabricate an explorer link where none exists).
• components/swap/SwapCard.tsx
  • Migrated off raw toast.error / toast.success (from sonner) to the shared notifyTxError / notifyTxSuccessLocal helpers, so styling and dismiss behavior match the rest of the app.
• components/TxToasts.test.tsx
  • Added a regression test asserting notifyTxError sets duration: Infinity.

Notes

• SwapCard's swap action doesn't currently submit a real on-chain transaction (no wallet/contract call, no tx hash produced), so its success toast intentionally has no explorer link — adding one would mean fabricating a link with no real destination. notifyTxSuccess (with explorer link) remains the canonical path once real execution is wired up here.
• One unrelated pre-existing test failure (components/swap/SwapRouteDisplay.test.tsx) was confirmed to fail on main as well (verified via git stash), so it's out of scope for this PR.

Testing

• npm test — all tests pass except the pre-existing unrelated failure noted above.