feat(mobile): multisig proposal, approval, and execution flow

frontend/mobile/README.md

@@ -327,3 +327,24 @@ opening links in the browser:
 deploying, verify with Apple's CDN
 (`https://app-site-association.cdn-apple.com/a/v1/app.veil.xyz`) and Google's
 [Digital Asset Links API](https://developers.google.com/digital-asset-links/tools/generator).
+## Multisig
+
+`/multisig` connects to a deployed M-of-N wallet
+(`contracts/multisig-wallet`) and runs the full lifecycle: an owner raises a
+transfer, owners approve it, and the approval that reaches the threshold
+executes it.
+
+There is no Execute button, because the contract has no `execute` entry point —
+`sign_transaction` performs the transfer in the same invocation that reaches the
+threshold. The screen names that approval for what it is ("Approve and execute")
+rather than implying a separate step that does not exist.
+
+Deployment stays on the desktop wizard; the contract address is stored under the
+same `veil_multisig_contract` key the web wallet uses. Point the screen at a
+network with `EXPO_PUBLIC_SOROBAN_RPC_URL` and `EXPO_PUBLIC_NETWORK_PASSPHRASE`
+(defaults to Soroban testnet).
+
+`lib/multisig.ts` holds the rules the screen applies before touching the chain —
+amount conversion, owner and duplicate-approval checks, and whether the next
+approval is the deciding one — so a rejection arrives immediately instead of as
+a contract panic after a fee.