feat!: refactor public config contract

[adriancmiranda]

Summary

Refactor the public configuration contract into nested tables for better structure and future extensibility.

Changes

• move cache-related options under cache
• move safety limits under safety
• move loading text under loading
• move format lists under image, video, and archive
• move video opener under video.open
• make integration toggles explicit with nested enable fields
• update internal consumers, README examples, and tests

Breaking change

Existing configs using the old flat keys must migrate to the new nested structure.

Old -> New

• cache_dir -> cache.dir
• cache_max_age_days -> cache.max_age_days
• max_file_size -> safety.max_file_size
• loading_text -> loading.text
• formats -> image.formats
• video_formats -> video.formats
• video_open -> video.open
• archive_formats -> archive.formats
• oil = true -> oil = { enable = true }
• telescope = true -> telescope = { enable = true }