chore(deps): bump file_picker 8→11 + transitive lifts

`flutter pub upgrade` lifts every transitive dep that the
constraint solver can reach: google_cloud 0.4.1→0.5.0,
googleapis_auth 2.3.0→2.3.1, hooks 1.0.3→2.0.0, objective_c
9.3.0→9.4.1 (plus xml 6.6.1).

Direct: file_picker 8.0.0 → 11.0.0. The major jump is the
move from instance API (`FilePicker.platform.pickFiles(...)`)
to static API (`FilePicker.pickFiles(...)`). Two call sites
updated (`lib/pages/flows.dart` for the run-flow form's
file input, `lib/widgets/fai_flow_output.dart` for the
flow-output "Save as" affordance) — both functionally
identical, just the surface that lost the `.platform.`
hop.

Locked behind external pins (no action possible from
Studio side this pass):
- grpc 4.2.0 → 5.1.0  ─┐
- protobuf 4.2.0 → 6.0.0 ┤── pinned by fai_client_sdk; a
                          │   major SDK bump is its own
                          │   piece of work (regenerated
                          │   bindings, possibly Studio
                          │   call-site shifts).
- characters / meta / vector_math / win32 / native_toolchain_c
   ─ all pinned by the Flutter SDK; lift with the next
     Flutter SDK upgrade.

dart analyze clean; flutter test green (12 tests pass).
Bumped pubspec to 0.48.1.

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

lib/widgets/fai_flow_output.dart

@@ -162,7 +162,7 @@ class _BytesView extends StatelessWidget {
 
   Future<void> _save(BuildContext context) async {
     final l = AppLocalizations.of(context)!;
-    final path = await FilePicker.platform.saveFile(
+    final path = await FilePicker.saveFile(
       dialogTitle: l.flowsOutputSaveAsTitle,
       fileName: _defaultFilename(),
     );