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/send.js

@@ -1,22 +1,41 @@
 // Send view: collect To, Amount, Token. Then go to confirmation.
 
-const { $, showError, hideError } = require("./helpers");
+const { $, showFlash } = require("./helpers");
 const { state, currentAddress } = require("../../shared/state");
 const { getProvider } = require("../../shared/balances");
 
+function updateSendBalance() {
+    const addr = currentAddress();
+    if (!addr) return;
+    const token = $("send-token").value;
+    if (token === "ETH") {
+        $("send-balance").textContent =
+            "Current balance: " + (addr.balance || "0") + " ETH";
+    } else {
+        const tb = (addr.tokenBalances || []).find(
+            (t) => t.address.toLowerCase() === token.toLowerCase(),
+        );
+        const symbol = tb ? tb.symbol : "?";
+        const bal = tb ? tb.balance || "0" : "0";
+        $("send-balance").textContent =
+            "Current balance: " + bal + " " + symbol;
+    }
+}
+
 function init(ctx) {
+    $("send-token").addEventListener("change", updateSendBalance);
+
     $("btn-send-review").addEventListener("click", async () => {
         const to = $("send-to").value.trim();
         const amount = $("send-amount").value.trim();
         if (!to) {
-            showError("send-error", "Please enter a recipient address.");
+            showFlash("Please enter a recipient address.");
             return;
         }
         if (!amount || isNaN(parseFloat(amount)) || parseFloat(amount) <= 0) {
-            showError("send-error", "Please enter a valid amount.");
+            showFlash("Please enter a valid amount.");
             return;
         }
-        hideError("send-error");
 
         // Resolve ENS if needed
         let resolvedTo = to;
@@ -26,13 +45,13 @@ function init(ctx) {
                 const provider = getProvider(state.rpcUrl);
                 const resolved = await provider.resolveName(to);
                 if (!resolved) {
-                    showError("send-error", "Could not resolve " + to);
+                    showFlash("Could not resolve " + to);
                     return;
                 }
                 resolvedTo = resolved;
                 ensName = to;
             } catch (e) {
-                showError("send-error", "Failed to resolve ENS name.");
+                showFlash("Failed to resolve ENS name.");
                 return;
             }
         }
@@ -53,4 +72,4 @@ function init(ctx) {
     $("btn-send-back").addEventListener("click", ctx.showAddressDetail);
 }
 
-module.exports = { init };
+module.exports = { init, updateSendBalance };