feat(studio): localised + explainer-rich Add module source dialog

The previous Add-source dialog was English-only and
operator-hostile — answered 'where do I install from?' without
explaining what a private module *is*. New version, DE + EN:

  - Title + intro + button labels routed through AppLocalizations
    ("Modul-Quelle hinzufügen" / "Installation fehlgeschlagen"
    instead of raw English strings).
  - 'How private modules work' explainer block (3 sentences):
    what a module is (`module.yaml` + WASM), how to package
    it (`fai pack <dir>` → .fai bundle), where to host it
    (any URL: own Forgejo / GitHub / S3), what verification the
    hub does (sha256 + signature against trust store).
  - Honest about the Studio gap: the dialog can install URLs +
    packed bundles, but unpacked source directories still
    require `fai install --link <path>` on the CLI. The
    example command lives in its own code-style box, selectable.
  - Layout: SingleChildScrollView'd so the explainer doesn't
    push the buttons off short viewports.

Studio bumped to 0.66.0; editor path-override pulls in 0.20.0.

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

lib/main.dart

@@ -27,7 +27,7 @@ import 'widgets/widgets.dart';
 /// Studio's own build version. Bump on every UI commit so the
 /// running app self-identifies — visible in the sidebar header
 /// and quick-glance proof that you're seeing the current build.
-const String kStudioVersion = '0.65.1';
+const String kStudioVersion = '0.66.0';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();