feat: live HubClient with generated proto bindings

Proto codegen wired in. tools/generate.sh wraps protoc against
../fai_platform/proto/, including the well-known types (Empty,
Struct, Timestamp) so they are generated alongside fai's own
messages — protobuf 4.x doesn't ship them as importable Dart
types, so we have to generate them ourselves.

HubClient now exposes typed methods backed by real RPCs:
- healthy() — Hub.Health probe (returns false on connect refused).
- listCapabilities() — HubAdmin.ListCapabilities.
- eventLog() — HubAdmin.EventLog with type filter.
- listApprovals() — HubAdmin.ListApprovals (status filter).
- approve() / reject() — HubAdmin.DecideApproval.

Generated bindings under lib/src/generated/ are excluded from
analyze (out of our style control). protoc_plugin pinned to
22.2.0 in tools/generate.sh comment because newer plugins
generate code against protobuf 6.x, while grpc 4.x still
constrains us to protobuf 4.x.

Bumps fai_dart_sdk 0.1.0 -> 0.2.0.

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

pubspec.yaml

@@ -10,6 +10,7 @@ environment:
 dependencies:
   grpc: ^4.0.0
   protobuf: ^4.0.0
+  fixnum: ^1.1.0
 
 dev_dependencies:
   lints: ^6.0.0