feat: implement web worker for stellar scanning

[bbkenny]

This PR moves the Stellar scanning logic to a Web Worker to improve UI responsiveness.

Key changes:

• Created src/workers/stellar-scanner.worker.ts to handle fetching and scanning announcements.
• Updated StellarReceive.tsx to use the Web Worker for background scanning.
• Improved error handling and worker lifecycle management (termination on unmount or re-scan).

Closes #3

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[bbkenny]

Hi @wraith-protocol, I've implemented the Web Worker for background scanning as requested in issue #3. This ensures the UI remains responsive even during heavy scanning tasks. Ready for review!

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[truthixify]

The Web Worker implementation is correct in shape (offloads scanAnnouncements from main thread, uses Soroban RPC ledger range probing, paginates via cursor, posts progress + result messages).

One blocker: the PR also adds a 16,456-line package-lock.json. This repo uses pnpm with pnpm-lock.yaml, so the npm lockfile would conflict and shouldn't be committed. Could you remove it?

rm package-lock.json
echo "package-lock.json" >> .gitignore  # if not already there
pnpm install  # regenerates pnpm-lock.yaml with any deps you added
git add -u .gitignore pnpm-lock.yaml
git commit --amend  # or new commit
git push --force-with-lease

Once the lockfile is sorted I will merge. Thanks @bbkenny.