feat: dismissable banner with session-storage memory (#81)

Adds a close button and session-scoped dismissal flag so the banner
can be acknowledged for the current run but reappears on the next
app launch if the fallback is still active — matches the #81
acceptance criterion.

- sessionStorage key survives page navigation within the run, is
  cleared on app restart.
- Graceful on storage quota errors.
- New `common.close` i18n key (FR: "Fermer", EN: "Close") used as
  the aria-label of the close button.

src/components/settings/TokenStoreFallbackBanner.tsx

@@ -1,11 +1,23 @@
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
-import { ShieldAlert } from "lucide-react";
+import { ShieldAlert, X } from "lucide-react";
 import { getTokenStoreMode, TokenStoreMode } from "../../services/authService";
 
+// Per-session dismissal flag. Kept in sessionStorage so the banner
+// returns on the next app launch if the fallback condition still
+// holds — this matches the acceptance criteria from issue #81.
+const DISMISS_KEY = "tokenStoreFallbackBannerDismissed";
+
 export default function TokenStoreFallbackBanner() {
   const { t } = useTranslation();
   const [mode, setMode] = useState<TokenStoreMode | null>(null);
+  const [dismissed, setDismissed] = useState<boolean>(() => {
+    try {
+      return sessionStorage.getItem(DISMISS_KEY) === "1";
+    } catch {
+      return false;
+    }
+  });
 
   useEffect(() => {
     let cancelled = false;
@@ -21,12 +33,22 @@ export default function TokenStoreFallbackBanner() {
     };
   }, []);
 
-  if (mode !== "file") return null;
+  if (mode !== "file" || dismissed) return null;
+
+  const dismiss = () => {
+    try {
+      sessionStorage.setItem(DISMISS_KEY, "1");
+    } catch {
+      // Ignore storage errors — the banner will simply hide for the
+      // remainder of this render cycle via state.
+    }
+    setDismissed(true);
+  };
 
   return (
     <div className="flex items-start gap-3 rounded-xl border border-amber-500/40 bg-amber-500/10 p-4">
       <ShieldAlert size={20} className="mt-0.5 shrink-0 text-amber-500" />
-      <div className="space-y-1 text-sm">
+      <div className="flex-1 space-y-1 text-sm">
         <p className="font-semibold text-[var(--foreground)]">
           {t("account.tokenStore.fallback.title")}
         </p>
@@ -34,6 +56,14 @@ export default function TokenStoreFallbackBanner() {
           {t("account.tokenStore.fallback.description")}
         </p>
       </div>
+      <button
+        type="button"
+        onClick={dismiss}
+        aria-label={t("common.close")}
+        className="shrink-0 text-[var(--muted-foreground)] hover:text-[var(--foreground)] transition-colors"
+      >
+        <X size={16} />
+      </button>
     </div>
   );
 }

src/i18n/locales/en.json

@@ -847,7 +847,8 @@
     "language": "Language",
     "total": "Total",
     "darkMode": "Dark mode",
-    "lightMode": "Light mode"
+    "lightMode": "Light mode",
+    "close": "Close"
   },
   "license": {
     "title": "License",

src/i18n/locales/fr.json

@@ -847,7 +847,8 @@
     "language": "Langue",
     "total": "Total",
     "darkMode": "Mode sombre",
-    "lightMode": "Mode clair"
+    "lightMode": "Mode clair",
+    "close": "Fermer"
   },
   "license": {
     "title": "Licence",