fix: implement proper view navigation stack

The popup UI had no navigation stack — all back buttons were hardcoded
to specific destinations. This meant navigating Main → Address →
Transaction → Settings → Back would go to Main instead of Transaction.

Changes:
- Add viewStack array to persisted state (survives popup close/reopen)
- Add pushCurrentView(), goBack(), clearViewStack() helpers in helpers.js
- goBack() pops the stack and shows the previous view; falls back to
  Main if the stack is empty; re-renders the wallet list when returning
  to the Main view
- All ctx.show*() wrappers in index.js now push the current view before
  navigating forward
- All back buttons across every view now use goBack() instead of
  hardcoded destinations
- Direct showView() calls for forward navigation (Send, Export Private
  Key) push before switching
- Stack is cleared on reset-to-root actions: adding a wallet, deleting
  the last wallet
- After wallet deletion with remaining wallets, stack is reset to
  [main] and settings is re-shown
- After transaction completion (Done button), stack is reset to the
  appropriate address context

Closes #134

src/popup/index.js

@@ -10,7 +10,14 @@ const {
 } = require("../shared/state");
 const { refreshPrices } = require("../shared/prices");
 const { refreshBalances } = require("../shared/balances");
-const { $, showView } = require("./views/helpers");
+const {
+    $,
+    showView,
+    setRenderMain,
+    pushCurrentView,
+    goBack,
+    clearViewStack,
+} = require("./views/helpers");
 const { applyTheme } = require("./theme");
 
 const home = require("./views/home");
@@ -58,15 +65,42 @@ async function doRefreshAndRender() {
 const ctx = {
     renderWalletList,
     doRefreshAndRender,
-    showAddWalletView: () => addWallet.show(),
-    showAddressDetail: () => addressDetail.show(),
-    showAddressToken: () => addressToken.show(),
-    showAddTokenView: () => addToken.show(),
-    showConfirmTx: (txInfo) => confirmTx.show(txInfo),
-    showReceive: () => receive.show(),
-    showTransactionDetail: (tx) => transactionDetail.show(tx),
-    showSettingsView: () => settings.show(),
-    showSettingsAddTokenView: () => settingsAddToken.show(),
+    showAddWalletView: () => {
+        pushCurrentView();
+        addWallet.show();
+    },
+    showAddressDetail: () => {
+        pushCurrentView();
+        addressDetail.show();
+    },
+    showAddressToken: () => {
+        pushCurrentView();
+        addressToken.show();
+    },
+    showAddTokenView: () => {
+        pushCurrentView();
+        addToken.show();
+    },
+    showConfirmTx: (txInfo) => {
+        pushCurrentView();
+        confirmTx.show(txInfo);
+    },
+    showReceive: () => {
+        pushCurrentView();
+        receive.show();
+    },
+    showTransactionDetail: (tx) => {
+        pushCurrentView();
+        transactionDetail.show(tx);
+    },
+    showSettingsView: () => {
+        pushCurrentView();
+        settings.show();
+    },
+    showSettingsAddTokenView: () => {
+        pushCurrentView();
+        settingsAddToken.show();
+    },
 };
 
 // Views that can be fully re-rendered from persisted state.
@@ -220,13 +254,15 @@ async function init() {
                 .getElementById("view-settings")
                 .classList.contains("hidden")
         ) {
-            renderWalletList();
-            showView("main");
+            goBack();
             return;
         }
+        pushCurrentView();
         settings.show();
     });
 
+    setRenderMain(renderWalletList);
+
     welcome.init(ctx);
     addWallet.init(ctx);
     home.init(ctx);