Split popup into one file per view

popup/index.js reduced to ~75 lines: loads state, builds a
shared context object, initializes all views, shows first screen.

Each view in popup/views/:
  helpers.js      — $(), showError, hideError, showView
  welcome.js      — welcome screen
  addWallet.js    — unified create/import recovery phrase
  importKey.js    — private key import
  home.js         — wallet list, total value, address derivation
  addressDetail.js — address view, token list, QR, copy
  send.js         — send form, ENS resolution, tx broadcast
  receive.js      — QR + copy
  addToken.js     — token lookup, common token picker
  settings.js     — RPC endpoint
  approval.js     — dApp approval (stub)

Views communicate via a ctx object with shared callbacks
(renderWalletList, showAddressDetail, doRefreshAndRender, etc).

src/popup/views/settings.js

@@ -0,0 +1,16 @@
+const { $, showView } = require("./helpers");
+const { state, saveState } = require("../../shared/state");
+
+function init(ctx) {
+    $("btn-save-rpc").addEventListener("click", async () => {
+        state.rpcUrl = $("settings-rpc").value.trim();
+        await saveState();
+    });
+
+    $("btn-settings-back").addEventListener("click", () => {
+        ctx.renderWalletList();
+        showView("main");
+    });
+}
+
+module.exports = { init };