fix: floor malformed allowedSites, fraudContracts and selectedToken entries (closes #362)

A stored allowedSites whose value was not a list rendered a working popup and then made every subsequent save fail silently, so the user operated a wallet that persisted nothing -- worse than a blank popup, which is at least visibly broken. fraudContracts and selectedToken had the same shape: a container floored by truthiness or not at all, while its entries were dereferenced. Entries are now floored as well as containers, following the idiom #311 established, and a failed save raises a persistent banner instead of vanishing into a swallowed rejection.

The per-field justifications that used to live in a hand-written header are replaced by a contract test that drives each field's hostile and falsy values through a real popup boot, so a claim about a field answers to the code rather than to prose. Its guarantee is stated narrowly and deliberately: no structural dereference on the code paths a wholly-corrupted profile takes, which is not every path a stored record takes. The paths it does not drive are named where the claim is made, and are tracked in #379.

src/popup/views/helpers.js

@@ -132,6 +132,38 @@ function updateDebugBanner(viewName) {
     }
 }
 
+// The banner shown when a save has failed, registered as the save-failure
+// reporter by src/popup/index.js.
+//
+// Persistent and not dismissable, unlike showFlash(): what it says is true
+// until the popup is closed, and a message that clears itself after two seconds
+// is how the user goes on operating a wallet that is persisting nothing
+// (https://git.eeqj.de/sneak/AutistMask/issues/362). It survives navigation
+// because it hangs off document.body rather than off a view.
+//
+// Created on demand rather than authored in index.html, the same way
+// updateDebugBanner() creates its own: it is absent from a popup where nothing
+// has failed, which is the state that must not need markup to be in.
+//
+// textContent, never innerHTML: `detail` carries an error message, which may
+// come from the browser's storage layer.
+function showSaveFailureBanner(detail) {
+    let banner = document.getElementById("save-failure-banner");
+    if (!banner) {
+        banner = document.createElement("div");
+        banner.id = "save-failure-banner";
+        banner.style.cssText =
+            "background:#c00;color:#fff;text-align:center;font-size:10px;padding:2px 4px;font-family:monospace;position:sticky;top:0;z-index:10000;";
+        document.body.prepend(banner);
+    }
+    const message = (detail && (detail.message || detail.problem)) || detail;
+    banner.textContent =
+        "NOT SAVED — AutistMask could not write to storage, so recent" +
+        " changes are not stored. Close and reopen the popup; if this keeps" +
+        " happening, do not rely on anything you change now." +
+        (message ? " (" + String(message) + ")" : "");
+}
+
 // Callback that renders a view being navigated BACK onto. Set once by
 // index.js via setBackRenderer(), which routes the view through the same
 // per-view render and data guards restoreView() uses.
@@ -544,6 +576,7 @@ module.exports = {
     showView,
     onViewLeave,
     updateDebugBanner,
+    showSaveFailureBanner,
     setBackRenderer,
     pushCurrentView,
     goBack,