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/helpers.js

@@ -0,0 +1,47 @@
+// Shared DOM helpers used by all views.
+
+const { DEBUG } = require("../../shared/wallet");
+
+const VIEWS = [
+    "welcome",
+    "add-wallet",
+    "import-key",
+    "main",
+    "address",
+    "send",
+    "receive",
+    "add-token",
+    "settings",
+    "approve",
+];
+
+function $(id) {
+    return document.getElementById(id);
+}
+
+function showError(id, msg) {
+    const el = $(id);
+    el.textContent = msg;
+    el.classList.remove("hidden");
+}
+
+function hideError(id) {
+    $(id).classList.add("hidden");
+}
+
+function showView(name) {
+    for (const v of VIEWS) {
+        const el = document.getElementById(`view-${v}`);
+        if (el) {
+            el.classList.toggle("hidden", v !== name);
+        }
+    }
+    if (DEBUG) {
+        const banner = document.getElementById("debug-banner");
+        if (banner) {
+            banner.textContent = "DEBUG / INSECURE (" + name + ")";
+        }
+    }
+}
+
+module.exports = { $, showError, hideError, showView };