feat(ui): surface depends_on blocking in status chip, chain label, and detail pane

Usability-Audit 2026-09-10 found three gaps versus the dependency-chain-display
spec: a Queued task blocked on an unmet task_dependencies edge rendered the same
chip as one about to run, a cross-list predecessor silently dropped the
"after #N" chip, and the detail pane showed nothing about a wait at all.

- TaskRowViewModel gains IsBlockedByDependency/DependencyWaitLabel (extrinsic,
  assigned by TasksIslandViewModel like the existing chain fields); IsQueued/
  IsWaiting/StatusChipClass/StatusLabel/StatusChipTooltip now account for it
  alongside the existing BlockedByTaskId planning-chain link.
- TasksIslandViewModel.ApplyDependencyWaitState computes the blocked flag and
  wait label per row; a new RefreshExternalDependencyCacheAsync backfills
  Number/Title/Status for predecessors not loaded into the current list's
  Items (reusing the already-open DbContext, no new hub method), so
  BuildAfterLabel/blocking both resolve a listenfremder Vorgänger correctly.
- New "waiting" chip style (IslandStyles.axaml) and locale keys (en+de);
  TaskHeaderBar shows a "Waiting on #N" line bound straight to the same row.

Tests: TasksIslandChainTests (blocked/unblocked chip, cross-list predecessor),
DetailsIslandDependencyWaitTests (new), TaskRowViewModelNotificationsTests
updated for the new notification set.

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -297,6 +297,18 @@
     <Setter Property="Foreground" Value="{StaticResource StatusQueuedBrush}" />
   </Style>
 
+  <!-- waiting → muted amber: a Queued task blocked on an unmet predecessor (a task_dependencies
+       edge, or the planning chain's BlockedByTaskId) — distinct from queued (unblocked, next up
+       to run) and from children (a parent waiting on its own subtasks). Mutually exclusive with
+       .queued on the same chip (IsQueued/IsWaiting), so no declaration-order fight between them. -->
+  <Style Selector="Border.chip.waiting">
+    <Setter Property="Background"  Value="#332F1A" />
+    <Setter Property="BorderBrush" Value="#4D451A" />
+  </Style>
+  <Style Selector="Border.chip.waiting > TextBlock">
+    <Setter Property="Foreground" Value="#C9A227" />
+  </Style>
+
   <!-- done → green (#6FA86B) -->
   <Style Selector="Border.chip.done">
     <Setter Property="Background"  Value="{StaticResource DoneTintBrush}" />