fix(runs): stop claiming the hub is unreachable on RPC-level errors

The runs monitor folded every load failure into 'hub not reachable',
contradicting the sidebar's green connected dot whenever the hub
answered but the RPC failed — most visibly against a pre-0.22 hub
whose version predates the ListInvocations RPC (UNIMPLEMENTED).

Classify the failure instead (top-level, unit-tested):
- UNIMPLEMENTED -> 'this view needs a newer hub version' with a
  doctor-page link (the update banner lives there)
- FAILED_PRECONDITION from the detached gate -> the regular
  feature-off empty state with the guide button
- UNAVAILABLE / DEADLINE_EXCEEDED / socket-level failures -> the
  honest 'hub not reachable' state (unchanged)
- everything else -> a load-failed state with the friendly error
  and a copyable detail box

The error view is a public callback-driven widget so the tests pump
each variant without a live hub. New DE+EN strings for the too-old
and load-failed states; grpcCodeOf/grpcMessageOf exposed from the
friendly-error mapper instead of duplicating the duck-typing.

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

lib/l10n/app_localizations_en.dart

@@ -3195,6 +3195,19 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get runsEmptyGuideButton => 'Open the guide';
 
+  @override
+  String get runsHubTooOldTitle => 'This view needs a newer hub version';
+
+  @override
+  String get runsHubTooOldHint =>
+      'The hub is connected, but its version does not know the runs monitor yet. Update the hub and the runs will appear here.';
+
+  @override
+  String get runsHubTooOldButton => 'Open Doctor';
+
+  @override
+  String get runsLoadFailedTitle => 'Runs could not be loaded';
+
   @override
   String get runsCancelButton => 'Cancel';