Add DEBUG mode with red banner and hardcoded mnemonic

When DEBUG=true: a sticky red "DEBUG / INSECURE" banner appears
at the top of all views, and the die button returns a hardcoded
test mnemonic instead of generating a random one.

src/popup/index.js

@@ -1,6 +1,10 @@
 // AutistMask popup UI — view management and event wiring
 const { Mnemonic } = require("ethers");
 
+const DEBUG = true;
+const DEBUG_MNEMONIC =
+    "cube evolve unfold result inch risk jealous skill hotel bulb night wreck";
+
 const VIEWS = [
     "lock",
     "welcome",
@@ -71,6 +75,7 @@ function makeStubAddress() {
 }
 
 function generateMnemonic() {
+    if (DEBUG) return DEBUG_MNEMONIC;
     const wallet = Mnemonic.fromEntropy(
         globalThis.crypto.getRandomValues(new Uint8Array(16)),
     );
@@ -239,6 +244,14 @@ function backFromWalletAdd() {
 
 // -- init --
 function init() {
+    if (DEBUG) {
+        const banner = document.createElement("div");
+        banner.textContent = "DEBUG / INSECURE";
+        banner.style.cssText =
+            "background:#c00;color:#fff;text-align:center;font-size:10px;padding:1px 0;font-family:monospace;position:sticky;top:0;z-index:9999;";
+        document.body.prepend(banner);
+    }
+
     if (!state.hasWallet) {
         showView("welcome");
     } else if (state.locked) {