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/wallet.js

@@ -2,12 +2,7 @@
 // All crypto delegated to ethers.js.
 
 const { Mnemonic, HDNodeWallet, Wallet } = require("ethers");
-
-const BIP44_ETH_BASE = "m/44'/60'/0'/0";
-
-const DEBUG = true;
-const DEBUG_MNEMONIC =
-    "cube evolve unfold result inch risk jealous skill hotel bulb night wreck";
+const { DEBUG, DEBUG_MNEMONIC, BIP44_ETH_PATH } = require("./constants");
 
 function generateMnemonic() {
     if (DEBUG) return DEBUG_MNEMONIC;
@@ -23,7 +18,7 @@ function deriveAddressFromXpub(xpub, index) {
 }
 
 function hdWalletFromMnemonic(mnemonic) {
-    const node = HDNodeWallet.fromPhrase(mnemonic, "", BIP44_ETH_BASE);
+    const node = HDNodeWallet.fromPhrase(mnemonic, "", BIP44_ETH_PATH);
     const xpub = node.neuter().extendedKey;
     const firstAddress = node.deriveChild(0).address;
     return { xpub, firstAddress };
@@ -39,7 +34,7 @@ function getSignerForAddress(walletData, addrIndex, decryptedSecret) {
         const node = HDNodeWallet.fromPhrase(
             decryptedSecret,
             "",
-            BIP44_ETH_BASE,
+            BIP44_ETH_PATH,
         );
         return node.deriveChild(addrIndex);
     }
@@ -51,9 +46,6 @@ function isValidMnemonic(mnemonic) {
 }
 
 module.exports = {
-    BIP44_ETH_BASE,
-    DEBUG,
-    DEBUG_MNEMONIC,
     generateMnemonic,
     deriveAddressFromXpub,
     hdWalletFromMnemonic,