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/addWallet.js

@@ -1,4 +1,4 @@
-const { $, showError, hideError, showView } = require("./helpers");
+const { $, showView, showFlash } = require("./helpers");
 const {
     generateMnemonic,
     hdWalletFromMnemonic,
@@ -6,13 +6,13 @@ const {
 } = require("../../shared/wallet");
 const { encryptWithPassword } = require("../../shared/vault");
 const { state, saveState } = require("../../shared/state");
+const { scanForAddresses } = require("../../shared/balances");
 
 function show() {
     $("wallet-mnemonic").value = "";
     $("add-wallet-password").value = "";
     $("add-wallet-password-confirm").value = "";
     $("add-wallet-phrase-warning").classList.add("hidden");
-    hideError("add-wallet-error");
     showView("add-wallet");
 }
 
@@ -25,16 +25,14 @@ function init(ctx) {
     $("btn-add-wallet-confirm").addEventListener("click", async () => {
         const mnemonic = $("wallet-mnemonic").value.trim();
         if (!mnemonic) {
-            showError(
-                "add-wallet-error",
-                "Please enter a recovery phrase or press the die to generate one.",
+            showFlash(
+                "Enter a recovery phrase or press the die to generate one.",
             );
             return;
         }
         const words = mnemonic.split(/\s+/);
         if (words.length !== 12 && words.length !== 24) {
-            showError(
-                "add-wallet-error",
+            showFlash(
                 "Recovery phrase must be 12 or 24 words. You entered " +
                     words.length +
                     ".",
@@ -42,34 +40,42 @@ function init(ctx) {
             return;
         }
         if (!isValidMnemonic(mnemonic)) {
-            showError(
-                "add-wallet-error",
-                "Invalid recovery phrase. Please check for typos.",
-            );
+            showFlash("Invalid recovery phrase. Check for typos.");
             return;
         }
         const pw = $("add-wallet-password").value;
         const pw2 = $("add-wallet-password-confirm").value;
         if (!pw) {
-            showError("add-wallet-error", "Please choose a password.");
+            showFlash("Please choose a password.");
             return;
         }
         if (pw.length < 8) {
-            showError(
-                "add-wallet-error",
-                "Password must be at least 8 characters.",
-            );
+            showFlash("Password must be at least 8 characters.");
             return;
         }
         if (pw !== pw2) {
-            showError("add-wallet-error", "Passwords do not match.");
+            showFlash("Passwords do not match.");
             return;
         }
-        hideError("add-wallet-error");
-        const encrypted = await encryptWithPassword(mnemonic, pw);
         const { xpub, firstAddress } = hdWalletFromMnemonic(mnemonic);
+        const duplicate = state.wallets.find(
+            (w) =>
+                w.type === "hd" &&
+                w.addresses[0] &&
+                w.addresses[0].address.toLowerCase() ===
+                    firstAddress.toLowerCase(),
+        );
+        if (duplicate) {
+            showFlash(
+                "This recovery phrase is already added (" +
+                    duplicate.name +
+                    ").",
+            );
+            return;
+        }
+        const encrypted = await encryptWithPassword(mnemonic, pw);
         const walletNum = state.wallets.length + 1;
-        state.wallets.push({
+        const wallet = {
             type: "hd",
             name: "Wallet " + walletNum,
             xpub: xpub,
@@ -78,11 +84,28 @@ function init(ctx) {
             addresses: [
                 { address: firstAddress, balance: "0.0000", tokenBalances: [] },
             ],
-        });
+        };
+        state.wallets.push(wallet);
         state.hasWallet = true;
         await saveState();
         ctx.renderWalletList();
         showView("main");
+
+        // Scan for used HD addresses beyond index 0.
+        showFlash("Scanning for addresses...", 30000);
+        const scan = await scanForAddresses(xpub, state.rpcUrl);
+        if (scan.addresses.length > 1) {
+            wallet.addresses = scan.addresses.map((a) => ({
+                address: a.address,
+                balance: "0.0000",
+                tokenBalances: [],
+            }));
+            wallet.nextIndex = scan.nextIndex;
+            await saveState();
+            ctx.renderWalletList();
+        }
+
+        ctx.doRefreshAndRender();
     });
 
     $("btn-add-wallet-back").addEventListener("click", () => {