Improve local demo and refresh documentation images

[Awuqing]

Summary

• refresh the demo with realistic backup tasks, records, storage targets, and cluster screenshots
• update English and Chinese README captions plus documentation showcase descriptions and image alt text
• make disk usage collection work on Windows
• preload each backup record's storage target and keep SQLite backups classified as full backups

Validation

• npm run typecheck
• npm run build
• go test ./internal/repository -run '^TestBackupRecordRepositoryQueries$' -count=1
• go test ./internal/service -run '^TestBackupExecutionServiceSQLiteBackupRemainsFull$' -count=1

Notes

A broader repository/service package run on Windows still encounters existing SQLite temporary-file cleanup failures in unrelated tests.