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/build.mjs

@@ -0,0 +1,15 @@
+// Bundles entry.js into the signing page's vendor directory: npm ci && node build.mjs
+import { build } from 'esbuild';
+
+await build({
+  entryPoints: ['entry.js'],
+  bundle: true,
+  format: 'iife',
+  globalName: 'RootSigningWC',
+  platform: 'browser',
+  target: 'es2020',
+  outfile: '../../../docs/root-signing/vendor/walletconnect.js',
+  minify: true,
+  legalComments: 'eof',
+  define: { 'process.env.NODE_ENV': '"production"', global: 'globalThis' },
+});