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/content/index.js

@@ -1,15 +1,17 @@
 // AutistMask content script — bridges between inpage (window.ethereum)
 // and the background service worker via extension messaging.
 
-// Inject the inpage script into the page's JS context
-const script = document.createElement("script");
-script.src = (typeof browser !== "undefined" ? browser : chrome).runtime.getURL(
-    "src/content/inpage.js",
-);
-script.onload = function () {
-    this.remove();
-};
-(document.head || document.documentElement).appendChild(script);
+// In Chrome (MV3), inpage.js runs as a MAIN-world content script declared
+// in the manifest, so no injection is needed here.  In Firefox (MV2), the
+// "world" key is not supported, so we inject via a <script> tag.
+if (typeof browser !== "undefined") {
+    const script = document.createElement("script");
+    script.src = browser.runtime.getURL("src/content/inpage.js");
+    script.onload = function () {
+        this.remove();
+    };
+    (document.head || document.documentElement).appendChild(script);
+}
 
 // Relay requests from the page to the background script
 window.addEventListener("message", (event) => {