Replace old encoder with new and delete

[GlebkaF]

Migrate to new Chain-based preset encoder/decoder, removing the old encoder.js and its adapter from the main application logic.

This PR refactors the preset handling by replacing the legacy encoder.js with a new lib/core/chain-converter.ts. The chain-converter.ts introduces a more structured Chain format for presets, enabling direct manipulation and conversion to/from QR strings and visualization data. The main application logic in pages/index.js now directly uses this new API, eliminating the need for the lib/core/adapter.ts (which has been deleted). A minimal encoder.js and a temporary adapter-for-tests.ts are retained for test compatibility.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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