Refactor popup into shared modules, wire up real ERC-20 tokens

Split popup/index.js (784 lines) into focused modules:
- shared/state.js: state management, storage persistence
- shared/wallet.js: mnemonic gen, HD derivation, signing
- shared/prices.js: price cache (5min TTL), USD formatting,
  value aggregation (address → wallet → total)
- shared/balances.js: ETH + ERC-20 balance cache (60s TTL),
  ENS lookup, token contract metadata lookup
- shared/vault.js: unchanged (libsodium encryption)
- shared/tokens.js: unchanged (token list + CoinDesk client)
- popup/index.js: view switching and event wiring only

Token tracking is now app-wide: trackedTokens stored in state,
balances fetched for all tracked tokens across all addresses.
Add Token now calls the real contract to read name/symbol/decimals.
Total portfolio value shown in 2x type on Home screen.

src/shared/wallet.js

@@ -0,0 +1,63 @@
+// Wallet operations: mnemonic generation, HD derivation, signing.
+// All crypto delegated to ethers.js.
+
+const { Mnemonic, HDNodeWallet, Wallet } = require("ethers");
+
+const BIP44_ETH_BASE = "m/44'/60'/0'/0";
+
+const DEBUG = true;
+const DEBUG_MNEMONIC =
+    "cube evolve unfold result inch risk jealous skill hotel bulb night wreck";
+
+function generateMnemonic() {
+    if (DEBUG) return DEBUG_MNEMONIC;
+    const m = Mnemonic.fromEntropy(
+        globalThis.crypto.getRandomValues(new Uint8Array(16)),
+    );
+    return m.phrase;
+}
+
+function deriveAddressFromXpub(xpub, index) {
+    const node = HDNodeWallet.fromExtendedKey(xpub);
+    return node.deriveChild(index).address;
+}
+
+function hdWalletFromMnemonic(mnemonic) {
+    const node = HDNodeWallet.fromPhrase(mnemonic, "", BIP44_ETH_BASE);
+    const xpub = node.neuter().extendedKey;
+    const firstAddress = node.deriveChild(0).address;
+    return { xpub, firstAddress };
+}
+
+function addressFromPrivateKey(key) {
+    const w = new Wallet(key);
+    return w.address;
+}
+
+function getSignerForAddress(walletData, addrIndex, decryptedSecret) {
+    if (walletData.type === "hd") {
+        const node = HDNodeWallet.fromPhrase(
+            decryptedSecret,
+            "",
+            BIP44_ETH_BASE,
+        );
+        return node.deriveChild(addrIndex);
+    }
+    return new Wallet(decryptedSecret);
+}
+
+function isValidMnemonic(mnemonic) {
+    return Mnemonic.isValidMnemonic(mnemonic);
+}
+
+module.exports = {
+    BIP44_ETH_BASE,
+    DEBUG,
+    DEBUG_MNEMONIC,
+    generateMnemonic,
+    deriveAddressFromXpub,
+    hdWalletFromMnemonic,
+    addressFromPrivateKey,
+    getSignerForAddress,
+    isValidMnemonic,
+};