feat(stellar): add payment link generation and consumption

[legend4tech]

Title

feat(stellar): add payment link generation and consumption

Description

This PR introduces the ability to generate and consume payment links on the Stellar network, solving the UX friction of sharing raw meta-addresses manually. Senders can now share a URL (or QR code) that deep-links directly into a pre-filled Send page.

Features

• Generate Payment Link: Added a new UI section to StellarReceive (visible after deriving keys) that allows recipients to generate a payment URL and QR code.
• Form Options: Senders can optionally pre-fill an amount, a memo, and an expires-at duration (defaults to 24h).
• Consumption Route (/pay): Added a /pay route that loads the Send page and populates the inputs based on the URL parameters.
• Validation & Tamper Resistance:
  • The Send form validates the exp timestamp. If the link is expired, it displays an error and disables the submit button.
  • Pre-filled inputs (recipient, amount, memo) are disabled on the Send page to ensure the sender sends exactly what was requested without accidental modification.
• Memo Support: Added native support for the memo field in StellarSend to append to the TransactionBuilder.
• E2E Testing: Includes a Playwright test suite (e2e/payment-link.spec.ts) validating the pre-fill behavior and expiration logic.

Design Decisions & Trade-offs

• URL Parameter Signing: Decided to skip signing URL parameters with the recipient's spending key for this iteration. Rationale: Keeps URLs short and simple. Since the sender always reviews the final transaction details in their wallet before signing, any tampering with the URL only alters the pre-filled form data, not the final authorized transaction.
• .wraith Name Lookup: Defaulted to using the raw meta-address for now, as on-chain .wraith name resolution context is not yet fully available for the Stellar chain in this codebase.

Screenshot

Testing

• Tested generating a link with combinations of amount, memo, and expiration.
• Verified QR code rendering.
• Verified /pay deep-link correctly pre-fills and locks down form inputs.
• Verified expired links are properly rejected.

closes #7

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[truthixify]

Payment link generation, the /pay route consumer, QR code, and Playwright e2e are exactly what issue #7 asked for. Conflicting after recent merges in StellarSend / StellarReceive.

Rebase:

git fetch origin
git rebase origin/develop
git push --force-with-lease

Keep your new e2e spec, playwright.config.ts, payment-link UI; layer your link generation form into develop's current StellarReceive (don't wipe the sponsored-withdrawal flow). Thanks @legend4tech.