feat(flows): sealed-aware flow list — hub sample flag, own dir, sample import

Three connection-truth fixes on the flow surface:

* The editor now lists the CONNECTED hub's flows: inside a sealed
  area Studio passes the instance's own flows dir
  (~/.chain/sealed/<slug>/data/flows) — previously the editor kept
  showing the shared hub's files whatever the connection, so a
  sealed area's list was simply wrong (and runSavedFlow hit the
  other hub's namespace).
* A connection switch replaces the editor state entirely (keyed by
  the sealed slug): an open buffer from one context never survives
  into the other — same privacy class as the switch race.
* Sample truth comes from the hub: listFlows' FlowSummary.sample
  (regenerated Dart SDK stubs) feeds the editor's sampleFlowNames;
  unknown (old hub / fetch failed) means no chips. Inside a sealed
  area the empty list offers the deliberate 'import example flows'
  action via chain flows import-samples against the instance's own
  dirs; the shared hub gets no such offer (it seeds samples itself,
  and a deliberate deletion is respected).

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

lib/data/hub.dart

@@ -1002,6 +1002,7 @@ class HubService {
             sizeBytes: f.sizeBytes.toInt(),
             requiredCapabilities: List<String>.from(f.requiredCapabilities),
             project: f.project,
+            sample: f.sample,
           ),
         )
         .toList()
@@ -1593,12 +1594,19 @@ class SavedFlow {
   /// project, then `general`).
   final String project;
 
+  /// True when the file still carries the bundled-sample header —
+  /// it arrived via the hub's sample import, not from an operator.
+  /// Old hubs without the wire field report false, so no chip is
+  /// ever shown on guesswork.
+  final bool sample;
+
   const SavedFlow({
     required this.name,
     required this.path,
     required this.sizeBytes,
     required this.requiredCapabilities,
     this.project = '',
+    this.sample = false,
   });
 }