refactor: extract embedded static assets

[lapismyt]

This refactors large embedded production string literals into static files loaded with include_str!.

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[Danil42Russia]

Why is this necessary, and what problem does it solve?

[lapismyt]

Why is this necessary, and what problem does it solve?

This is not required for runtime behavior; it is a maintainability refactor.

These strings are production assets, but they were embedded as very large Rust literals. That makes the Rust modules harder to read, makes diffs noisy, and makes editing/reviewing the actual asset content awkward. Moving them into files keeps the code focused on behavior while letting the assets live in their native formats: HTML as HTML, licenses as text files, base templates as template files, and the default config as a BOC64 asset.

The binary behavior stays the same because the files are still embedded at compile time with include_str!. There is no runtime file lookup or packaging change. I also verified the extracted DEFAULT_CONFIG byte-for-byte against the previous inline string, so this should be behavior-preserving.