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/state.js

@@ -1,5 +1,7 @@
 // State management and extension storage persistence.
 
+const { DEFAULT_RPC_URL, DEFAULT_BLOCKSCOUT_URL } = require("./constants");
+
 const storageApi =
     typeof browser !== "undefined"
         ? browser.storage.local
@@ -9,7 +11,9 @@ const DEFAULT_STATE = {
     hasWallet: false,
     wallets: [],
     trackedTokens: [],
-    rpcUrl: "https://eth.llamarpc.com",
+    rpcUrl: DEFAULT_RPC_URL,
+    blockscoutUrl: DEFAULT_BLOCKSCOUT_URL,
+    lastBalanceRefresh: 0,
 };
 
 const state = {
@@ -24,6 +28,8 @@ async function saveState() {
         wallets: state.wallets,
         trackedTokens: state.trackedTokens,
         rpcUrl: state.rpcUrl,
+        blockscoutUrl: state.blockscoutUrl,
+        lastBalanceRefresh: state.lastBalanceRefresh,
     };
     await storageApi.set({ autistmask: persisted });
 }
@@ -36,6 +42,9 @@ async function loadState() {
         state.wallets = saved.wallets || [];
         state.trackedTokens = saved.trackedTokens || [];
         state.rpcUrl = saved.rpcUrl || DEFAULT_STATE.rpcUrl;
+        state.blockscoutUrl =
+            saved.blockscoutUrl || DEFAULT_STATE.blockscoutUrl;
+        state.lastBalanceRefresh = saved.lastBalanceRefresh || 0;
     }
 }