Add shareable gift links for fast onboarding

Create a #claim=<code> link from Settings → "Gift this wallet" (gated
behind a reveal + warning, with a QR): the seed (and passphrase, if any)
are encoded as BIP39 entropy in base64url in the URL fragment, which
never reaches a server. Opening such a link loads that wallet and shows
a welcome → balance → firm "back up your recovery phrase" flow, then
drops into the wallet. The claim code is scrubbed from the address bar
on load.

Bearer by design — whoever opens the link controls the funds — so the
creator UI warns to keep amounts small. New strings translated into all
19 locales.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 23 insertions, 2 deletions

diff --git a/src/wallet.js b/src/wallet.js
index edacc05..08f5d9e 100644
--- a/src/wallet.js
+++ b/src/wallet.js
@@ -9,9 +9,9 @@
 // transactions, only the (txid, vout, value, address) of each UTXO.
 
 import { HDKey } from '@scure/bip32';
-import { generateMnemonic, mnemonicToSeedSync, validateMnemonic } from '@scure/bip39';
+import { generateMnemonic, mnemonicToSeedSync, validateMnemonic, mnemonicToEntropy, entropyToMnemonic } from '@scure/bip39';
 import { wordlist } from '@scure/bip39/wordlists/english';
-import { hex } from '@scure/base';
+import { hex, base64urlnopad } from '@scure/base';
 import { sha256 } from '@noble/hashes/sha256';
 import * as btc from '@scure/btc-signer';
 import { p2wpkh } from '@scure/btc-signer/payment';
@@ -1185,6 +1185,27 @@ export function utxoId(u) {
   return `${u.txid}:${u.vout}`;
 }
 
+// Gift links: encode a wallet (12-word seed as BIP39 entropy, plus the optional
+// passphrase) compactly for the #claim= fragment as base64url, and decode it
+// back to { mnemonic, passphrase }. Returns null if the code isn't valid.
+export function encodeGift(mnemonic, passphrase = '') {
+  let code = base64urlnopad.encode(mnemonicToEntropy(mnemonic.trim().replace(/\s+/g, ' '), wordlist));
+  if (passphrase) code += '~' + base64urlnopad.encode(new TextEncoder().encode(passphrase));
+  return code;
+}
+
+export function decodeGift(code) {
+  try {
+    const [seedPart, passPart] = code.split('~');
+    const mnemonic = entropyToMnemonic(base64urlnopad.decode(seedPart), wordlist);
+    if (!validateMnemonic(mnemonic, wordlist)) return null;
+    const passphrase = passPart ? new TextDecoder().decode(base64urlnopad.decode(passPart)) : '';
+    return { mnemonic, passphrase };
+  } catch {
+    return null;
+  }
+}
+
 function firstUnused(chain) {
   const u = chain.find((a) => !a.used);
   return u ? u.index : chain.length;