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/send.js

@@ -7,6 +7,7 @@ const {
     escapeHtml,
     renderAddressHtml,
     attachCopyHandlers,
+    goBack,
 } = require("./helpers");
 const { state, currentAddress } = require("../../shared/state");
 let ctx;
@@ -250,11 +251,7 @@ function init(_ctx) {
     $("btn-send-back").addEventListener("click", () => {
         $("send-token").classList.remove("hidden");
         $("send-token-static").classList.add("hidden");
-        if (state.selectedToken) {
-            ctx.showAddressToken();
-        } else {
-            ctx.showAddressDetail();
-        }
+        goBack();
     });
 }