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/constants.js

@@ -1,6 +1,12 @@
+const DEBUG = true;
+const DEBUG_MNEMONIC =
+    "cube evolve unfold result inch risk jealous skill hotel bulb night wreck";
+
 const ETHEREUM_MAINNET_CHAIN_ID = "0x1";
 
-const DEFAULT_RPC_URL = "https://eth.llamarpc.com";
+const DEFAULT_RPC_URL = "https://ethereum-rpc.publicnode.com";
+
+const DEFAULT_BLOCKSCOUT_URL = "https://eth.blockscout.com/api/v2";
 
 const BIP44_ETH_PATH = "m/44'/60'/0'/0";
 
@@ -15,8 +21,11 @@ const ERC20_ABI = [
 ];
 
 module.exports = {
+    DEBUG,
+    DEBUG_MNEMONIC,
     ETHEREUM_MAINNET_CHAIN_ID,
     DEFAULT_RPC_URL,
+    DEFAULT_BLOCKSCOUT_URL,
     BIP44_ETH_PATH,
     ERC20_ABI,
 };