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/Views/Islands/TaskRowView.axaml

@@ -142,7 +142,8 @@
                     Classes.children="{Binding Status, Converter={StaticResource EqStatus}, ConverterParameter=WaitingForChildren}"
                     Classes.done="{Binding Status, Converter={StaticResource EqStatus}, ConverterParameter=Done}"
                     Classes.error="{Binding Status, Converter={StaticResource EqStatus}, ConverterParameter=Failed}"
-                    Classes.queued="{Binding Status, Converter={StaticResource EqStatus}, ConverterParameter=Queued}">
+                    Classes.queued="{Binding IsQueued}"
+                    Classes.waiting="{Binding IsWaiting}">
               <TextBlock Text="{Binding StatusLabel}"/>
             </Border>