fix(report): preserve directory stats in JSON export

[ShivamB25]

Summary

• include apparent size, disk usage, and item count in directory JSON metadata
• preserve aggregate statistics for depth-truncated and summarized directories when reports are reloaded
• keep full report recomputation behavior unchanged and retain aggregate cutoffs under file filtering
• add JSON export/import regression coverage, including empty directories

Verification

• go test -v -covermode=count ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.2 run ./report/... ./cmd/gdu/app/... ./cmd/gdu/...
• CLI export/import smoke tests for a depth-limited report

Closes #325

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.76%. Comparing base (1868609) to head (5e91e01).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
+ Coverage   85.69%   85.76%   +0.06%     
==========================================
  Files          55       55              
  Lines        6243     6272      +29     
==========================================
+ Hits         5350     5379      +29     
  Misses        689      689              
  Partials      204      204              

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[ShivamB25]

lgtm.