Fix fill-password: wrong host key, and silent failure

Two bugs made 'Fill saved password' do nothing:

1. HOST MISMATCH. Saving used location.hostname at submit time (e.g.
   accounts.google.com); filling derived the host from the app's
   configured URL (mail.google.com). The Keychain lookup always found
   nothing for any service with a separate auth domain. Fixed by storing
   the save-time hostname in App.saved_host and reading it back at fill
   time, falling back to default_scope for entries saved before this.

2. SILENT FAILURE. Every error path in the fillpw sentinel called only
   eprintln!, which is invisible to the user. A feature that fails silently
   is indistinguishable from one that is not wired up. Fixed by emitting a
   fill-error event; the shell shows it as a toast in the nav column, which
   is left of the stage and therefore always above native views.

src/App.tsx

@@ -32,6 +32,7 @@ export default function App() {
   const [downloads, setDownloads] = useState<Download[]>([]);
   const [offer, setOffer] = useState<PasswordOffer | null>(null);
   const [saved, setSaved] = useState<string | null>(null);
+  const [fillError, setFillError] = useState<string | null>(null);
 
   const stageRef = useRef<HTMLDivElement>(null);
   const booted = useRef(false);
@@ -147,6 +148,11 @@ export default function App() {
       }),
       // A login was submitted. The password is held in Rust; this only asks.
       listen<PasswordOffer>("password-offer", (e) => setOffer(e.payload)),
+      // Fill failed — show the reason rather than doing nothing silently.
+      listen<string>("fill-error", (e) => {
+        setFillError(e.payload);
+        setTimeout(() => setFillError(null), 5000);
+      }),
 
       listen<{ id: number; name: string; path: string }>("download-started", (e) =>
         setDownloads((d) => [
@@ -322,6 +328,21 @@ export default function App() {
         </Dialog>
       )}
 
+      {fillError && (
+        /* Positioned within the nav column (left of the stage), so it is
+           always above native views. The stage starts at x = collapsed ? rail
+           : PANEL; this toast stays well within that. */
+        <div
+          role="alert"
+          style={{ width: collapsed ? rail : 240 }}
+          className="fixed bottom-4 left-0 z-[80] px-3"
+        >
+          <div className="rounded-lg border border-red-200 bg-red-50 px-3 py-2 text-[12px] leading-snug text-red-700 shadow-md dark:border-red-900 dark:bg-red-950/60 dark:text-red-300">
+            {fillError}
+          </div>
+        </div>
+      )}
+
       {settingsOpen && (
         <Settings
           config={config}