test: drive the Settings screen in a browser and guard every popup element id (closes #229)

README.md

@@ -165,6 +165,23 @@ fixtures in `tests/e2e/network.js`, so the run is deterministic and fully
 offline; unrecognised outbound requests are reported as failures rather than
 silently allowed.
 
+It also covers the **Settings screen**, which holds the densest run of element
+id lookups in the codebase and where one wrong id leaves the whole popup blank
+rather than only degrading Settings: that the screen renders populated — the
+About well and the wallet list are read back, so a `show()` that stopped early
+is caught rather than merely a view that failed to appear — that the four Token
+Spam Protection controls are real checkboxes defaulted on, and that the theme
+and network selectors offer exactly the choices `src/shared/networks.js` and
+`index.html` define. What the selectors persist is then driven through the UI to
+`dark` and `sepolia` and reasserted after a popup reopen before being restored
+the same way; neither value is its selector's first `<option>`, so neither can
+be read back from the markup with no JavaScript having run. One spam filter is
+likewise toggled off and back on across a reopen each way, which exercises the
+change handler, `saveState()`, `loadState()` and the assignment `init()` makes.
+Each group of these assertions records a coverage key and a final case demands
+the exact set, so a section that silently stopped running reddens the suite
+instead of quietly shrinking it.
+
 It also covers the confirmation screen, for both a native ETH send and an ERC-20
 send: Send disabled while the fee estimate is in flight, enabled once it lands,
 the fee block quoting the expected cost and the reserve separately, the distinct
@@ -400,6 +417,24 @@ the one-time pull of the pinned ~800MB Playwright layer, and well under a minute
 once that layer is cached. `e2e-firefox` about 1m05s cold, and it caches its
 Firefox and geckodriver downloads the same way.
 
+### Element id guard (part of `make check`)
+
+`tests/popupElementIds.test.js` asserts statically that every element id the
+popup looks up — `$("...")`, `document.getElementById("...")`,
+`showError()`/`hideError()`, and the `view-<name>` a literal `showView("...")`
+resolves to — exists in `src/popup/index.html`, and that `index.html` defines no
+id twice. A wrong id is valid JavaScript naming a defined function, so neither
+jest (node environment, no DOM) nor a linter objects to it; at runtime `$()`
+returns `null` and the next property access throws, which inside a view's
+`init()` aborts the rest of `src/popup/index.js` `init()` and leaves the popup
+blank.
+
+It runs with no browser, so unlike the e2e suites it fits inside `make check`,
+and it covers every view rather than the ones some test happens to open. It only
+sees literal arguments: a call like `$(containerId)` is invisible to it, and a
+lookup naming the wrong existing element is valid by construction. Both of those
+are the browser suites' job.
+
 ## Rationale
 
 Common popular EVM wallets have become bloated with swap UIs, portfolio