feat(studio): optional pinned-key field in the store-add form

The module-store manager's 'Add a store' form gains an optional PEM
public-key field; when set it pins that publisher key to the store
(per-store signing trust). Localized DE/EN.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_localizations_en.dart

@@ -833,6 +833,14 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get storesManagerAddShort => 'Add';
 
+  @override
+  String get storesManagerPinnedKeyLabel =>
+      'Pinned public key — PEM (optional)';
+
+  @override
+  String get storesManagerPinnedKeyHint =>
+      'Signed modules from this store verify against this key instead of the built-in vendor key. Paste a PEM public key.';
+
   @override
   String get storesManagerNotAvailable => 'Not available yet';