feat(stellar): add Stellar deep-link helpers

[uboho-prog]

close #74

PR description
Title
Add Stellar Expert deep-link helpers and surface "View on stellar.expert" links

Summary
Centralize Stellar explorer URL generation and add consistent "View on stellar.expert" links in the Stellar send/receive UI.

Changes

Added: [stellarAccountUrl](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/), [stellarContractUrl](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/), [stellarAssetUrl](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/), [stellarOperationUrl](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) helpers in [explorer.ts](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/).
Updated: [StellarSend.tsx](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) — added "View on stellar.expert" links for announcer contract, stealth address, and tx hash.
Updated: [StellarReceive.tsx](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) — added "View on stellar.expert" links for stealth addresses, withdraw/registration txs, and registry contract.

Motivation
Some Stellar identifiers were linked in one place but not others. This centralizes explorer URL generation and makes Stellar links consistent and discoverable across the demo UI.

How to test

Run typecheck and dev server:

In app (Stellar / Testnet):
    Send: perform a test send; confirm result panel shows the stealth address and tx with clickable "View on stellar.expert" links opening the Stellar Expert testnet pages.
    Receive: derive keys, (optionally) register, scan for payments; confirm each stealth row shows a "View on stellar.expert" link for the stealth address and any txs; registry contract link appears next to "On-Chain Registration".
    Verify external links open in a new tab and include [rel="noopener"](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/).

Files modified

[explorer.ts](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/)
[StellarSend.tsx](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/)
[StellarReceive.tsx](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/)
[task74.md](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) (deleted)

Acceptance checklist

Centralized helpers: [explorer.ts](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) exposes account/tx/contract/asset/operation helpers for Stellar.
UI links added: "View on stellar.expert" present where Stellar identifiers appear in Send/Receive.
Network-aware: links use the configured [STELLAR_NETWORK.explorerUrl](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) (testnet in this branch).
Accessibility/security: external links include [rel="noopener"](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/) and [target="_blank"](https://cuddly-yodel-x56jjwj6jvqgcvg47.github.dev/).
Typecheck: tsc --noEmit passes locally.

Notes

Branch: deep-link/helper
No functional logic changes beyond adding links and helpers.

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[truthixify]

Two things before merge:

1. Drop package-lock.json (25,919 lines npm lockfile). This repo uses pnpm. Use pnpm install to regenerate the pnpm lockfile if you added deps.
2. Conflicts in StellarSend.tsx and StellarReceive.tsx. Quick rebase:

git fetch origin
git rebase origin/develop
rm package-lock.json
git add -u
git commit --amend
git push --force-with-lease

The explorer.ts helpers look good.