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>

src/ClaudeDo.Data/Git/GitService.cs

@@ -124,6 +124,20 @@ public sealed class GitService
         return await GetDiffAsync(worktreePath, ct);
     }
 
+    /// <summary>
+    /// Diff between two commits, run in any repo that can reach them. Used to view a
+    /// task's changes after its worktree has been merged away (the commits survive on
+    /// the target branch even though the worktree directory and branch ref are gone).
+    /// </summary>
+    public async Task<string> GetCommitRangeDiffAsync(string repoDir, string baseCommit, string headCommit, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, stderr) = await RunGitAsync(repoDir,
+            ["diff", $"{baseCommit}..{headCommit}"], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git diff {baseCommit}..{headCommit} failed (exit {exitCode}): {stderr}");
+        return stdout;
+    }
+
     public async Task<string> DiffStatAsync(string worktreePath, string baseCommit, string headCommit, CancellationToken ct = default)
     {
         var (exitCode, stdout, stderr) = await RunGitAsync(worktreePath,