Add total portfolio value, cached prices and balances

Total USD value displayed in 2x type above wallet list on Home.
Value aggregation: getAddressValueUsd (ETH + all tokens) →
getWalletValueUsd → getTotalValueUsd. Price API cached for 5
minutes, balance fetches cached for 60 seconds. Both caches
are app-wide — repeated calls to refreshPrices/refreshBalances
are no-ops within the TTL.

src/popup/index.html

@@ -173,6 +173,9 @@
 
             <!-- ============ MAIN VIEW: ALL WALLETS & ADDRESSES ============ -->
             <div id="view-main" class="view hidden">
+                <!-- total portfolio value -->
+                <div id="total-value" class="text-2xl font-bold mb-2"></div>
+
                 <!-- wallet list -->
                 <div id="wallet-list"></div>