[codex] Improve gallery correctness and robustness

[haltakov]

Summary

• Preserves unknown gallery JSON fields while keeping hero variant size maps validated.
• Fixes init/migration edge cases, deterministic directory skipping, title capitalization, CLI help naming, and non-interactive build confirmations.
• Consolidates gallery.json writes, surfaces Astro build output, adds scan pruning, improves thumbnail failure accounting, and replaces Axios usage with fetch.
• Adds Node engine metadata and focused regression tests for build, migration, scan, and thumbnail failure handling.

Validation

• yarn workspace @simple-photo-gallery/common build
• yarn workspace simple-photo-gallery check
• yarn workspace @simple-photo-gallery/theme-modern lint && yarn workspace @simple-photo-gallery/theme-modern format
• yarn workspace simple-photo-gallery test

[chatgpt-codex-connector]

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