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

Nothing exercised the Settings view in a browser, and jest runs in the
node environment with no DOM, so the densest run of $("...") lookups in
the codebase was unverified at runtime. A wrong id is valid JavaScript
naming a defined function: $() returns null and the next property access
throws, which inside a view's init() aborts the rest of the popup's
init() and leaves every screen blank.

Two halves, because they catch different things.

The e2e suite (tests/e2e/run.js) gains seven cases between the address
removal and dust threshold sections. They assert the About well and the
wallet list were actually written — show() populates those near its end,
only the debug well and the debug-mode checkbox follow, so reading them
back proves show() ran through to there rather than just far enough to
unhide the section — that the four Token Spam Protection controls are
real input[type=checkbox] elements 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 asserted by a round trip through
NON-DEFAULT values: they are driven to dark and sepolia, the popup is
closed and reopened, both are read back, and both are then restored the
same way and reasserted after a second reopen. Neither value is the
first <option> of its <select>, which is the point — the first option is
what the DOM reports with no JavaScript having run at all, so asserting
it would pass just as happily against a Settings screen that assigned
nothing. One spam filter is likewise toggled off and back on across a
reopen each way. Those round trips run the change handler, saveState(),
loadState() and the assignments show() and init() make, rather than only
looking at the screen. Each group records a coverage key and a final
case demands the exact set, so a section that silently stopped running
reddens the suite instead of shrinking it.

show() no longer wraps its settings-network lookup in if (networkSelect),
and neither does init(): a null there was silently skipped, which is
exactly the failure this change exists to make loud.

tests/popupElementIds.test.js is the general half and needs no browser,
so jest picks it up and it runs in make check: every literal id reached
through $(), document.getElementById(), showError()/hideError() and
showView() must exist in src/popup/index.html, no id in index.html may
be defined twice, and the scan asserts it found the code and the markup
so it cannot pass by covering nothing. Only literal arguments are
resolvable statically; $(containerId) and a lookup naming the wrong
existing element are the browser suites' job, and README says so.

Demonstrated against four deliberate breaks. A typo'd id in settings.js
reddens both halves, the e2e run reporting "pageerror: Cannot set
properties of null (setting 'checked')" against its first test. A
handler bound to the wrong but existing element passes the static guard
and reddens only the new functional case. A typo in a view no browser
suite opens reddens only the static guard. Deleting either persisted
value assignment in settings.js — the theme one in init(), the network
one in show() — reddens the selector round trip and nothing else, each
one on its own.

README.md

@@ -159,6 +159,23 @@ 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
@@ -362,6 +379,24 @@ 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