feat(ui): richer diff viewer + surface child roadblocks on parents

- UnifiedDiffParser detects added/deleted/renamed/binary files; diff
  modal shows a file list, binary/empty placeholders, and can diff a
  merged task by commit range after its worktree is gone
- DetailsIslandViewModel flags children needing attention (failed,
  cancelled, awaiting review, or with roadblocks) on the parent
- GitService gains worktree head-commit/range support; planning chain,
  merge orchestration, and session manager tweaks with updated tests
- refresh app/installer/worker icons

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/ClaudeDo.Worker.Tests/Services/TaskMergeServiceTests.cs

@@ -531,6 +531,8 @@ public class TaskMergeServiceTests : IDisposable
         Assert.Equal(TaskStatus.Done, updated!.Status);
         var wt = await new WorktreeRepository(ctx).GetByTaskIdAsync(task.Id);
         Assert.Equal(WorktreeState.Merged, wt!.State);
+        // Approve removes the worktree so merged worktrees don't pile up.
+        Assert.False(Directory.Exists(wtCtx.WorktreePath));
     }
 
     [Fact]