Fix search to match substrings instead of exact text

[Vlad1k88]

Summary

• Fixed matches() in lib/store.js to use String.prototype.includes() instead of strict equality (===), so search returns all notes whose text contains the term rather than only exact matches.

Test plan

• npm test — all 3 tests pass, including the two that were previously failing (search finds every note that contains the term, search finds a single containing note)

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