Sign the root over WalletConnect (#83)

* Sign the root over WalletConnect

Wallet 1.3.0's DApp browser never delivers injected-provider requests, so the page could
not get a signature from any installed wallet. WalletConnect is the route installed
wallets answer: on the phone the page opens the wallet by its pezkuwiwallet://wc link,
elsewhere it shows a code for the wallet to scan, then asks for polkadot_signMessage on
<Bytes>payload</Bytes>. Measured over a real session on the 1.3.0 signing code: the
signature verifies under the "substrate" context, which is what the thresholds count.

The injected provider stays as a second button for wallets where it works. The
WalletConnect bundle is built from scripts/root/wc (pinned versions, node build.mjs).

* Open the wallet by a fixed link, not the peer's redirect

The redirect in a WalletConnect session's peer metadata is supplied by whichever wallet
connects, so following it let a connecting peer navigate the signing page anywhere,
javascript: included.

scripts/root/wc/entry.js

@@ -0,0 +1,10 @@
+// WalletConnect for the root signing page: the sign client and a QR encoder, as window.RootSigningWC.
+// Buffer and process are what WalletConnect's dependencies expect a browser to have.
+import { Buffer } from 'buffer';
+import process from 'process';
+if (typeof globalThis.Buffer === 'undefined') globalThis.Buffer = Buffer;
+if (typeof globalThis.process === 'undefined') globalThis.process = process;
+if (!globalThis.process.version) globalThis.process.version = 'v18.0.0';
+
+export { SignClient } from '@walletconnect/sign-client';
+export { toDataURL as qrDataUrl } from 'qrcode';