fix: implement proper view navigation stack (#146)

## Summary

Fixes the view stack pop bug where pressing Back in Settings (or any view) always returned to Main instead of the previous view.

Closes [issue #134](#134)

## Problem

The popup UI had no navigation stack. Every back button was hardcoded to a specific destination (usually Main). The reported path:

> Main → Address → Transaction → Settings (gear icon) → Back

...would go to Main instead of returning to the Transaction view.

## Solution

Implemented a proper view navigation stack (like iOS) as already described in the README:

- **`viewStack`** array added to persisted state — survives popup close/reopen
- **`pushCurrentView()`** — pushes the current view name onto the stack before any forward navigation
- **`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 Main
- **`clearViewStack()`** — resets the stack for root transitions (e.g., after adding/deleting a wallet)

### What Changed

1. **helpers.js** — Added navigation stack functions (`pushCurrentView`, `goBack`, `clearViewStack`, `setRenderMain`)
2. **state.js** — Added `viewStack` to persisted state
3. **index.js** — All `ctx.show*()` wrappers now push before navigating forward; gear button uses stack for toggle behavior
4. **All view back buttons** — Replaced hardcoded destinations with `goBack()` (settings, addressDetail, addressToken, transactionDetail, send, receive, addToken, confirmTx, addWallet, settingsAddToken, deleteWallet, export-privkey)
5. **Direct `showView()` forward navigations** — Added `pushCurrentView()` calls before `showView("send")` in addressDetail, addressToken, and home; before `showView("export-privkey")` in addressDetail; before `deleteWallet.show()` in settings
6. **Reset-to-root transitions** — `clearViewStack()` called after adding a wallet (all 3 import types), after deleting the last wallet, and after transaction completion (Done button)

### Navigation Paths Verified

- **Main → Settings → Back** → returns to Main ✓
- **Main → Address → Settings → Back** → returns to Address ✓
- **Main → Address → Transaction → Settings → Back** → returns to Transaction ✓ (the reported bug)
- **Main → Address → Token → Send → ConfirmTx → Back → Back → Back → Back** → unwinds correctly through each view back to Main ✓
- **Main → Address → Token → Transaction → Settings → Back** → returns to Transaction ✓
- **Settings → Add Wallet → (add) → Main** → stack cleared, fresh root ✓
- **Settings → Delete Wallet → Back** → returns to Settings ✓
- **Settings → Delete Wallet → (confirm)** → stack reset to [main], settings shown ✓
- **Address → Send → ConfirmTx → (broadcast) → SuccessTx → Done** → stack reset, returns to address context ✓
- **Popup close/reopen** → viewStack persisted, back navigation still works ✓

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #146
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

src/popup/views/helpers.js

@@ -75,6 +75,43 @@ function showView(name) {
     }
 }
 
+// Callback to re-render the main/home view when navigating back to it.
+// Set once by index.js via setRenderMain().
+let _renderMain = null;
+
+function setRenderMain(fn) {
+    _renderMain = fn;
+}
+
+// Push the current view onto the navigation stack so goBack() can
+// return to it.  Call this before any forward navigation.
+function pushCurrentView() {
+    if (state.currentView) {
+        state.viewStack.push(state.currentView);
+    }
+}
+
+// Pop the navigation stack and show the previous view.  If the stack
+// is empty, fall back to the main (home) view.
+function goBack() {
+    let target;
+    if (state.viewStack.length > 0) {
+        target = state.viewStack.pop();
+    } else {
+        target = "main";
+    }
+    if (target === "main" && _renderMain) {
+        _renderMain();
+    }
+    showView(target);
+}
+
+// Clear the entire navigation stack (used when resetting to root,
+// e.g. after adding or deleting a wallet).
+function clearViewStack() {
+    state.viewStack = [];
+}
+
 let flashTimer = null;
 
 function clearFlash() {
@@ -380,6 +417,10 @@ module.exports = {
     showError,
     hideError,
     showView,
+    setRenderMain,
+    pushCurrentView,
+    goBack,
+    clearViewStack,
     showFlash,
     flashCopyFeedback,
     balanceLine,