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/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "pezkuwi-root-signing-walletconnect",
+  "private": true,
+  "type": "module",
+  "description": "Builds docs/root-signing/vendor/walletconnect.js: node build.mjs",
+  "dependencies": {
+    "@walletconnect/sign-client": "2.23.9",
+    "buffer": "6.0.3",
+    "process": "0.11.10",
+    "qrcode": "1.5.4"
+  },
+  "devDependencies": {
+    "esbuild": "0.28.1"
+  }
+}