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/popup/views/helpers.js

@@ -1,6 +1,11 @@
 // Shared DOM helpers used by all views.
 
-const { DEBUG } = require("../../shared/wallet");
+const { DEBUG } = require("../../shared/constants");
+const {
+    formatUsd,
+    getPrice,
+    getAddressValueUsd,
+} = require("../../shared/prices");
 
 const VIEWS = [
     "welcome",
@@ -37,6 +42,7 @@ function showView(name) {
             el.classList.toggle("hidden", v !== name);
         }
     }
+    clearFlash();
     if (DEBUG) {
         const banner = document.getElementById("debug-banner");
         if (banner) {
@@ -45,4 +51,58 @@ function showView(name) {
     }
 }
 
-module.exports = { $, showError, hideError, showView };
+let flashTimer = null;
+
+function clearFlash() {
+    if (flashTimer) {
+        clearTimeout(flashTimer);
+        flashTimer = null;
+    }
+    $("flash-msg").textContent = "";
+}
+
+function showFlash(msg, duration = 2000) {
+    clearFlash();
+    $("flash-msg").textContent = msg;
+    flashTimer = setTimeout(() => {
+        $("flash-msg").textContent = "";
+        flashTimer = null;
+    }, duration);
+}
+
+function balanceLine(symbol, amount, price) {
+    const qty = amount.toFixed(4);
+    const usd = price ? formatUsd(amount * price) : "";
+    return (
+        `<div class="flex text-xs">` +
+        `<span class="flex justify-between shrink-0" style="width:42ch">` +
+        `<span>${symbol}</span>` +
+        `<span>${qty}</span>` +
+        `</span>` +
+        `<span class="text-right text-muted flex-1">${usd}</span>` +
+        `</div>`
+    );
+}
+
+function balanceLinesForAddress(addr) {
+    let html = balanceLine(
+        "ETH",
+        parseFloat(addr.balance || "0"),
+        getPrice("ETH"),
+    );
+    for (const t of addr.tokenBalances || []) {
+        const bal = parseFloat(t.balance || "0");
+        if (bal === 0) continue;
+        html += balanceLine(t.symbol, bal, getPrice(t.symbol));
+    }
+    return html;
+}
+
+module.exports = {
+    $,
+    showError,
+    hideError,
+    showView,
+    showFlash,
+    balanceLinesForAddress,
+};