fix(git): carry the pathspec into GetBranchDiffAsync's fallback

src/ClaudeDo.Data/Git/GitService.cs

@@ -139,16 +139,20 @@ public sealed class GitService
             throw new InvalidOperationException($"git commit failed (exit {exitCode}): {stderr}");
     }
 
-    public async Task<string> GetDiffAsync(string worktreePath, CancellationToken ct = default)
+    public async Task<string> GetDiffAsync(
+        string worktreePath, IReadOnlyList<string>? paths = null, CancellationToken ct = default)
     {
-        var (exitCode, stdout, stderr) = await RunGitAsync(worktreePath,
-            ["diff", "HEAD"], ct);
+        var args = new List<string> { "diff", "HEAD" };
+        AppendPathFilter(args, paths);
+        var (exitCode, stdout, stderr) = await RunGitAsync(worktreePath, args, ct);
         if (exitCode != 0)
             throw new InvalidOperationException($"git diff HEAD failed (exit {exitCode}): {stderr}");
         // If nothing staged vs HEAD, try the index (untracked is never in diff)
         if (string.IsNullOrWhiteSpace(stdout))
         {
-            var (e2, s2, _) = await RunGitAsync(worktreePath, ["diff", "--cached"], ct);
+            var cachedArgs = new List<string> { "diff", "--cached" };
+            AppendPathFilter(cachedArgs, paths);
+            var (e2, s2, _) = await RunGitAsync(worktreePath, cachedArgs, ct);
             if (e2 == 0) return s2;
         }
         return stdout;
@@ -167,8 +171,11 @@ public sealed class GitService
         var (exitCode, stdout, _) = await RunGitAsync(worktreePath, args, ct);
         if (exitCode == 0 && !string.IsNullOrWhiteSpace(stdout))
             return stdout;
-        // Fallback: whatever the worktree has vs HEAD (uncommitted only).
-        return await GetDiffAsync(worktreePath, ct);
+        // Fallback: whatever the worktree has vs HEAD (uncommitted only). The same pathspec has to
+        // come along — a filter makes an empty ranged diff the common case (the caller asked for
+        // paths this range never touched), and an unfiltered fallback would answer that with the
+        // whole worktree diff, the exact opposite of what was requested.
+        return await GetDiffAsync(worktreePath, paths, ct);
     }
 
     /// <summary>

src/ClaudeDo.Ui/ViewModels/Modals/DiffViewerViewModel.cs

@@ -165,7 +165,7 @@ public sealed partial class DiffViewerViewModel : ViewModelBase
                 ? await _git.GetCommitRangeDiffAsync(WorktreePath, BaseRef, HeadCommit, ct: ct)
                 : BaseRef is not null
                     ? await _git.GetBranchDiffAsync(WorktreePath, BaseRef, ct: ct)
-                    : await _git.GetDiffAsync(WorktreePath, ct);
+                    : await _git.GetDiffAsync(WorktreePath, ct: ct);
         }
         catch (Exception ex)
         {