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/importKey.js

@@ -1,4 +1,4 @@
-const { $, showError, hideError, showView } = require("./helpers");
+const { $, showView, showFlash } = require("./helpers");
 const { addressFromPrivateKey } = require("../../shared/wallet");
 const { encryptWithPassword } = require("../../shared/vault");
 const { state, saveState } = require("../../shared/state");
@@ -7,7 +7,6 @@ function show() {
     $("import-private-key").value = "";
     $("import-key-password").value = "";
     $("import-key-password-confirm").value = "";
-    hideError("import-key-error");
     showView("import-key");
 }
 
@@ -15,34 +14,30 @@ function init(ctx) {
     $("btn-import-key-confirm").addEventListener("click", async () => {
         const key = $("import-private-key").value.trim();
         if (!key) {
-            showError("import-key-error", "Please enter your private key.");
+            showFlash("Please enter your private key.");
             return;
         }
         let addr;
         try {
             addr = addressFromPrivateKey(key);
         } catch (e) {
-            showError("import-key-error", "Invalid private key.");
+            showFlash("Invalid private key.");
             return;
         }
         const pw = $("import-key-password").value;
         const pw2 = $("import-key-password-confirm").value;
         if (!pw) {
-            showError("import-key-error", "Please choose a password.");
+            showFlash("Please choose a password.");
             return;
         }
         if (pw.length < 8) {
-            showError(
-                "import-key-error",
-                "Password must be at least 8 characters.",
-            );
+            showFlash("Password must be at least 8 characters.");
             return;
         }
         if (pw !== pw2) {
-            showError("import-key-error", "Passwords do not match.");
+            showFlash("Passwords do not match.");
             return;
         }
-        hideError("import-key-error");
         const encrypted = await encryptWithPassword(key, pw);
         const walletNum = state.wallets.length + 1;
         state.wallets.push({
@@ -57,6 +52,8 @@ function init(ctx) {
         await saveState();
         ctx.renderWalletList();
         showView("main");
+
+        ctx.doRefreshAndRender();
     });
 
     $("btn-import-key-back").addEventListener("click", () => {