Token auto-discovery, tx history, balance polling, EIP-6963, UI overhaul

Major changes:
- Fetch token balances and tx history from Blockscout API (configurable)
- Remove manual token discovery (discoverTokens) in favor of Blockscout
- HD address gap scanning on mnemonic import
- Duplicate mnemonic detection on wallet add
- EIP-6963 multi-wallet discovery + selectedAddress updates in inpage
- Two-tier balance refresh: 10s while popup open, 60s background
- Fix $0.00 flash before prices load (return null when no prices)
- No-layout-shift: min-height on total value element
- Aligned balance columns (42ch address width, consistent USD column)
- All errors use flash messages instead of off-screen error divs
- Settings gear in global title bar, add-wallet moved to settings pane
- Settings wells with light grey background, configurable Blockscout URL
- Consistent "< Back" buttons top-left on all views
- Address titles (Address 1.1, 1.2, etc.) on main and detail views
- Send view shows current balance of selected asset
- Clickable affordance policy added to README
- Shortened mnemonic backup warning
- Fix broken background script constant imports

src/shared/prices.js

@@ -37,12 +37,10 @@ function formatUsd(amount) {
 }
 
 function getAddressValueUsd(addr) {
+    if (!prices.ETH) return null;
     let total = 0;
     const ethBal = parseFloat(addr.balance || "0");
-    const ethPrice = prices.ETH;
-    if (ethPrice) {
-        total += ethBal * ethPrice;
-    }
+    total += ethBal * prices.ETH;
     for (const token of addr.tokenBalances || []) {
         const tokenBal = parseFloat(token.balance || "0");
         if (tokenBal > 0 && prices[token.symbol]) {
@@ -53,6 +51,7 @@ function getAddressValueUsd(addr) {
 }
 
 function getWalletValueUsd(wallet) {
+    if (!prices.ETH) return null;
     let total = 0;
     for (const addr of wallet.addresses) {
         total += getAddressValueUsd(addr);
@@ -61,6 +60,7 @@ function getWalletValueUsd(wallet) {
 }
 
 function getTotalValueUsd(wallets) {
+    if (!prices.ETH) return null;
     let total = 0;
     for (const wallet of wallets) {
         total += getWalletValueUsd(wallet);