feat: add multisig contract functionality and WASM upload script

[fadesany]

• Implemented multisig contract interactions in multisig.ts, including deployment, initialization, transaction proposal, and signing.
• Added fallback service worker for offline support in fallback-hUSKnVm1DRDhF4S2kqLZN.js.
• Introduced workbox library for enhanced caching strategies in workbox-4754cb34.js.
• Created script for uploading multisig WASM contract to Stellar testnet in upload_multisig_wasm.mjs.

closes #233

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[Miracle656]

Nice work landing a working m-of-n multisig contract plus the wizard/queue UI. A few things to fix before merge.

🟠 Committed build artifacts

Please remove generated/build files from the diff — they shouldn't be tracked:

• frontend/wallet/public/sw.js (the +1 -101 mangling), public/workbox-4754cb34.js, public/fallback-hUSKnVm1DRDhF4S2kqLZN.js
• frontend/wallet/tsconfig.tsbuildinfo

These are regenerated by the PWA build; committing them creates noise and merge conflicts. Add them to .gitignore if they aren't already and drop them from this PR.

🟠 Contract: propose_transaction has no authorization

Anyone can call propose_transaction — it doesn't check the caller is an owner or require_auth(). Even though execution needs threshold approvals, an unauthenticated party can spam the proposal store (state-bloat / griefing) and pre-seed to/amount. Gate it: require the caller to be an owner and require_auth() them.

🟠 No events emitted

propose_transaction / sign_transaction / execution emit no events, so wraith (and any indexer) can't track multisig activity. Please env.events().publish(...) on propose, approve, and execute.

🟢 Looks good

• Init validates threshold > 0 and owners.len() >= threshold, blocks double-init.
• sign_transaction correctly requires owner + require_auth, dedupes approvals, sets executed before transfer, and reverts atomically on transfer failure.

Once the artifacts are dropped and propose_transaction is gated, I'll re-review. CI hasn't run the full suite yet — I've approved the workflow so typecheck/build will execute.