feat: implement `SEP24Dialog` component for hosted deposit/withdrawal flows

[rayeberechi]

Summary

SEP-24 defines the interactive anchor deposit and withdrawal protocol. This is the most complex component in the kit — it opens an anchor's hosted iframe flow inside a Radix UI Dialog.

Background

SEP-24 flow:

1. Fetch the anchor's stellar.toml to discover the TRANSFER_SERVER_SEP0024 URL.
2. Call GET /info to list supported assets and operations.
3. Call POST /transactions/deposit/interactive or POST /transactions/withdraw/interactive with a JWT (obtained via SEP-10).
4. Open the returned url in an iframe inside a modal.
5. Poll GET /transaction?id= until the transaction reaches a terminal state (completed, error, refunded).

Implementation Steps

1. Create src/components/transaction/SEP24Dialog.tsx.
2. Props:
   • anchorDomain: string — e.g. "myanchor.io".
   • operation: 'deposit' | 'withdraw'.
   • assetCode: string.
   • jwt: string — SEP-10 JWT; obtaining it is out of scope for this component.
   • onSuccess: (transactionId: string) => void.
   • onError: (error: Error) => void.
   • onClose: () => void.
   • open: boolean.
   • className?: string.
3. Use @stellar/stellar-sdk's StellarToml.Resolver.resolve() to fetch the TOML.
4. Use Radix UI Dialog.Root / Dialog.Content for the modal shell.
5. Render the anchor URL in an <iframe sandbox="allow-scripts allow-same-origin allow-forms allow-popups" /> — the sandbox attribute is mandatory for security.
6. Implement polling with setInterval (clear on unmount). Poll every 5 seconds.
7. Create src/hooks/useSEP24Transaction.ts to encapsulate the polling logic.
8. Export SEP24Dialog and useSEP24Transaction from src/index.ts.
9. Write tests:
   • Renders Dialog when open=true.
   • Does not render iframe when open=false.
   • Calls onSuccess when polling returns completed.
   • Calls onError when polling returns error.

Security Considerations

• Always use sandbox on the iframe.
• Never pass the JWT as a URL query parameter — use the anchor's POST endpoint.
• Validate that the polled transaction ID matches the one returned by the initial POST.

Definition of Done

• TOML resolution implemented.
• Radix UI Dialog used for modal shell.
• Iframe rendered with correct sandbox attributes.
• Polling implemented in useSEP24Transaction hook.
• onSuccess / onError / onClose callbacks all wired up.
• Security considerations addressed.
• All tests pass.
• No TypeScript errors in strict mode.