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/prices.js

@@ -0,0 +1,79 @@
+// Price fetching with 5-minute cache, USD formatting, value aggregation.
+
+const { getTopTokenPrices } = require("./tokens");
+
+const PRICE_CACHE_TTL = 300000; // 5 minutes
+
+const prices = {};
+let lastFetchedAt = 0;
+
+async function refreshPrices() {
+    const now = Date.now();
+    if (now - lastFetchedAt < PRICE_CACHE_TTL) return;
+    try {
+        const fetched = await getTopTokenPrices(25);
+        Object.assign(prices, fetched);
+        lastFetchedAt = now;
+    } catch (e) {
+        // prices stay stale on error
+    }
+}
+
+function getPrice(symbol) {
+    return prices[symbol] || null;
+}
+
+function formatUsd(amount) {
+    if (amount === null || amount === undefined || isNaN(amount)) return "";
+    if (amount === 0) return "$0.00";
+    if (amount < 0.01) return "< $0.01";
+    return (
+        "$" +
+        amount.toLocaleString("en-US", {
+            minimumFractionDigits: 2,
+            maximumFractionDigits: 2,
+        })
+    );
+}
+
+function getAddressValueUsd(addr) {
+    let total = 0;
+    const ethBal = parseFloat(addr.balance || "0");
+    const ethPrice = prices.ETH;
+    if (ethPrice) {
+        total += ethBal * ethPrice;
+    }
+    for (const token of addr.tokenBalances || []) {
+        const tokenBal = parseFloat(token.balance || "0");
+        if (tokenBal > 0 && prices[token.symbol]) {
+            total += tokenBal * prices[token.symbol];
+        }
+    }
+    return total;
+}
+
+function getWalletValueUsd(wallet) {
+    let total = 0;
+    for (const addr of wallet.addresses) {
+        total += getAddressValueUsd(addr);
+    }
+    return total;
+}
+
+function getTotalValueUsd(wallets) {
+    let total = 0;
+    for (const wallet of wallets) {
+        total += getWalletValueUsd(wallet);
+    }
+    return total;
+}
+
+module.exports = {
+    prices,
+    refreshPrices,
+    getPrice,
+    formatUsd,
+    getAddressValueUsd,
+    getWalletValueUsd,
+    getTotalValueUsd,
+};