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/settings.js

@@ -1,12 +1,76 @@
-const { $, showView } = require("./helpers");
+const { $, showView, showFlash } = require("./helpers");
 const { state, saveState } = require("../../shared/state");
+const { ETHEREUM_MAINNET_CHAIN_ID } = require("../../shared/constants");
+const { log } = require("../../shared/log");
 
 function init(ctx) {
     $("btn-save-rpc").addEventListener("click", async () => {
-        state.rpcUrl = $("settings-rpc").value.trim();
+        const url = $("settings-rpc").value.trim();
+        if (!url) {
+            showFlash("Please enter an RPC URL.");
+            return;
+        }
+        showFlash("Testing endpoint...");
+        try {
+            const resp = await fetch(url, {
+                method: "POST",
+                headers: { "Content-Type": "application/json" },
+                body: JSON.stringify({
+                    jsonrpc: "2.0",
+                    id: 1,
+                    method: "eth_chainId",
+                    params: [],
+                }),
+            });
+            const json = await resp.json();
+            if (json.error) {
+                log.errorf("RPC validation error:", json.error);
+                showFlash("Endpoint returned error: " + json.error.message);
+                return;
+            }
+            if (json.result !== ETHEREUM_MAINNET_CHAIN_ID) {
+                showFlash(
+                    "Wrong network (expected mainnet, got chain " +
+                        json.result +
+                        ").",
+                );
+                return;
+            }
+        } catch (e) {
+            log.errorf("RPC validation fetch failed:", e.message);
+            showFlash("Could not reach endpoint.");
+            return;
+        }
+        state.rpcUrl = url;
         await saveState();
+        showFlash("Saved.");
     });
 
+    $("btn-save-blockscout").addEventListener("click", async () => {
+        const url = $("settings-blockscout").value.trim();
+        if (!url) {
+            showFlash("Please enter a Blockscout API URL.");
+            return;
+        }
+        showFlash("Testing endpoint...");
+        try {
+            const resp = await fetch(url + "/stats");
+            if (!resp.ok) {
+                showFlash("Endpoint returned HTTP " + resp.status + ".");
+                return;
+            }
+        } catch (e) {
+            log.errorf("Blockscout validation failed:", e.message);
+            showFlash("Could not reach endpoint.");
+            return;
+        }
+        state.blockscoutUrl = url;
+        await saveState();
+        showFlash("Saved.");
+    });
+
+    $("btn-main-add-wallet").addEventListener("click", ctx.showAddWalletView);
+
     $("btn-settings-back").addEventListener("click", () => {
         ctx.renderWalletList();
         showView("main");